MSTISLAV ROSTROPOVICH has nothing if not audacity - whether as personality, cellist, pianist, political dissident or, in this case, budding conductor. When he decides to go for broke, nothing musical is beyond his aspirations. His success at mesmerizing legions of followers is such - and his talent is of sufficient magnitude - that when yet another occasion appears to call for reaffirmation of his prowess, he seems consumed with intrepid daring.

Thus Rostropovich's first recording of a symphony in his flourishing midlife conducting career could never have been just, say, an hour of Beethoven, or even an hour and a half of Mahler. That's how other conductors do it.

Instead, there arrives a weighty set of seven Angel discs on which Rostropovich conducts no fewer than 5 hours and 40 minutes of Tchaikovsky. That includes the six numbered symphonies, plus the epic "Manfred" symphony-tone poem. All were done in a herculean three weeks of sessions last fall with the London Philharmonic Orchestra. Thus, little more than month before his Oct. 4 debut as the National Symphony's music director, comes the most substantial recorded evidence yet of what musical life is likely to be like here under the man the National Symphony baldly promotes as "the world's greatest musician."

And to compound the audacity of this phonographic event, these records go into direct competition with another of the handful of complete recordings of these six symphonies - under Antal Dorati, whose job Rostropovich has taken and who, as principal guest conductor, is advertised in the orchestra's brochure merely as an "inspired" leader.

So at last we have direct evidence for comparison in the ongoing debate over "Slava" versus Dorati. These recordings provide something more tangible than memory of live events on which to weigh the orchestra management's confidence and that, at least, of the more vocal segment of the concert audience - that the National is moving up a notch with the ascension of the charismatic Rostropovich. Further, they test the fears of others that Rostropovich has been oversold and the less charismatic Dorati underestimated.

It seems a fair match. Rostropovich's sneer showmanship on the podium is stripped away, as is Dorati's sober, match-of-fact demeanor. The orchestras are fairly evenly matched (Dorati conducts the London Symphony on Mercury). Both men have solid reputations as Tchiakovsky performers. And the reproduction is fine on both.

Each set has interpretive strengths and weaknesses, but even the weaknesses are helpful in judging the contrast of styles. And the records are helpful in pinpointing the coincidence that each conductor suffers from a different interpretive hangup that spoils some otherwise worthy performances because of damage to the music's forward momentum. In the case of Rostropovich, it's an occasional playing of a slow section so slowly or retarding the pace so much that the train of musical thought gets lost. It's not that slavish attention to metronomic markings is essential a practice he quite rightly deplores in his opening notes of the excellent program booklet, but it is essential that the line not be so distorted that the basic pulse is lost. It's like the difference between having your car in neutral and having it in park the minimum pulse is a hard target to hit, and Rostropovich sometimes fails.

The Dorati hungup is simpler, and one with which Washington audiences became familiar. It's an unpredictable inclination, when he doesn't particularly have his heart in something, to give it a deadpan runthrough without much attention to nicities of phrasing.But, as he shows on these discs, Dorati can be an interpreter of great eloquence.

The Rostropovich style is more individual, if not necessarily for a cellist, he gives more attention to the lower-pitched instruments than any other conductor I can think of. When the main line is in the tuba or the bassoon, higher-pitched instruments are reined in to put the line in clear relief. As a result, the overall sound of its inordinately dark, with the higher winds, in particular, less audible than usual.

The dominant mood in Rostropovich's Tchaikovsky on the records is less the "passionate" mood that has been compared by some to the interpretation of the late Serge Koussevitsky than a dark, grim mood that works best when the movement is melancholy and despondent and the pace is not too slow (for example, the last movement of the "Pathetique"). It functions less well in the waltzes, even in minor keys. Movements like these are put there to provide at least a little relief from the grimness, and Rostropovich oftens denies us this.

And it is in these movements that Dorati, a Tchaikovsky ballet specialist, is often at his most captivating. He favors a lighter, purer, more richly colored kind of sound, and that, combined with his sense of rhythmic flexibility, gives the waltzes especially a grace and variety of mood that Rostropovich eschews. In the early, less well-known and more cheerful symphonies, Dorati's tempos are brisker than Rostropovich's, but in the later, often-performed ones, there is less tempo variation (aside from those slow sections) than you might expert.

Finally, just to confound us, each conductor turns in one first-rate performance that breaks these general guidelines. Dorati's extraordinary urgent, well-proportioned version of the tricky 5th is one of the best around. The orchestra plays at its most brilliant, and that is very good. The famous wind themes of the second movement are meltingly lyrical. The demanding rhythmic rapids of the first movement are ridden with precision and the interpretation in general is a remarkable fusing of the conflicting demands for passion, on the one hand, and clarity, on the other.

Rostropovich's brilliant 2d symphony (the "Little Russian") is a model of fleet, but understated elegence. The strings are uncharacteristically light, and articulate with real virtnosity. Here the momentum far excells Dorati's in a work where he should logically have come out on top.

A major disappointment is Rostropovich's "Manfred," one of Tchaikovsky's finest creations and one in need of a modern recording to match the drama of Toscanini's famous performance on mono. Here, perhaps, along with the 5th, is Rostropovich's sacrifice of the music's momentum most damaging.

There is no clear-cut winner in this match. Three performances seem first class: Rostropovich's 2d, Dorati's 1st and Dorati's 5th. Five are good, with individual movements that are excellent: Dorati's 2nd, and Rostropovich's 1st, 3rd, 4th and 6th. Five are failures: Dorati's 3rd, 4th and 6th (all victims of that deadpan manner) and Rostropovich in the 5th and in "Manfred."

Three first-class performances are not to be sneezed at, but one hoped for more from both conductors.

And as if to put this point in perspective, a performance of the "Pathetique" by Herbert von Karajan and Berlin Philharmonic on Deutshe Grammophon arrived recently in which that sometimes erratic conductor eclipses any performance on either of these sets. This "Pathetique" (following on several less-successful ones by Karajan) is a knockout. The movements are smashingly dramatic without any loss of clarity or cohesion. The orchestra, arguably the world's finest, is brilliant. And the sound is superb. After about 15 hours of listening to lesser performances, it was like a cold shower. And in its example it is a standard that should not be forgotten as we follow the course of our orchestra here