Elvis Costello and John Hiatt could be runts from the same litter. Both are avid students of rock. Both employ relatively sparse instrumentation, showing a rare (and justified) confidence in their lyrics. And both possess a mean streak a mile wide, their antisocial hostility and caustic humor barely held in check. Yet Costello's name is much more familiar than Hiatt's, even though Hiatt has been around a bit longer.

One reason for this state of affairs is that Costello has the full support of his record company, as evidenced by the customized label, the in-house liner notes and the extensive advertising support afforded his new album, "Taking Liberties." Hiatt has not been so fortunate. His new album, "Two Bit Monsters," was released a couple of months ago with little fanfare and advertising support. What small noise there has been has come from those who expressed bewilderment that the same radio stations that fawn over the British Costello do not seem to have room for an American artist with many of the same attributes.

This is not to complain about Costello's asension to stardom. "Taking Liberties," a 20-song compilation of B-sides, Britis album cuts, unreleased masters and other odds and ends, bears witness to Costello's incredible prolificacy and the respect he has earned as one of rock's most charismatic and gripping singer-songwriters. And if the album lacks cohesion (the material was recorded at different times over the past four years), it offers impressive proof that Costello's throwaways are better than most artists plums.

"Taking Liberties, (Columbia JC36839) catches Elvis in some of his quirkier, more playful moods; with a lyricist of his inventiveness, the random discards can be greater fun than the more studied effort. Some of the most appealing tracks are those in which Elvis steps furthest out of his petulant character, such as on "Radio Sweetheart," a 1977 foray into countrified rock, and "Stranger in the House," which works both as a tribute to C&W great George Jones and on its own as a steel-guitar-infused, cry-in-your-beer country song. Similarly, Costello pulls off a show tune (his reading of Rodgers and Hart's "My Funny Valentine" is faithful and heartfelt), an R&B dance tune (Van McCoy's "Getting Mighty Crowded") and bouncy bubblegum ()Crawling to the U.S.A." parodies the American pop of artists like Freddy Cannon).

Even the two out-takes from Elvis' previous album, "Black and White World" and "Clowntime is Over," are slower, less cluttered and easier to take than the overstated versions on "Get Happy!" And one can trace the development of the manic, high energy R&B style that dominates Costello's recent work through such cuts here as "I Don't Want to Go to) Chelsea," with its guitar fixture neatly copped from The Seeds' "Pushin' Too Hard," and the raucous "Clean Money."

There are some true expendables, but also a few gems, easily on a par with anything Costello has recorded. "Talking in the Dard" is a bittersweet, Kinks-like plaint; the mid-tempo "Tiny Steps" highlights Costello's continuing obsession with models; "Big Tears" is a Dylanesque, Farfisa organ-dominated rocker that closes with a flourish of screaming voice (Costello) and guitar (Mick Jones of The Clash); and "Girls Talk," which can't match the immediate musicality of Dave Edmunds' version but ultimately suports the adage that an artist is generaly the best interpreter of his own material.

On his first two albums for Epic in 1973-75 (now out of print), John Hiatt was a rocker in sheeps clothing, his offbeat sense of humor obscured by bland Nashville production. Although he still might benefit from a band or producer with more punch, Hiatt has happily given in to his rock 'n' roll muse on his two recent releases for MCA. 'is nasal vocal quality and trenchant humor have fostered the comparisons to Costello, but Hiatt was writing and singing that way while Costello was still pumping gas. But "Two Bit Monsters," (MCA 5123) like "Taking Liberties," draws from such modes as soul ("Down in Front"), Mo-town ("I Spy [for the FBI]"), ("Back to Normal"). And Hiatt demonstrates Costello's ability to turn a phrase like a knife in songs such as "Face the Nation" -- an indictment of spoon-fed TV news that musically recalls the Rolling Stones' "Paint It Black" -- and the sardonic "Cop Party," about some policemen out for a good time.

Many of Hiatt's most engaging songs (again, like Costello's) provide a commentary on the battle of the sexes. "Back to the War" even employs miitary terminology, while the beautifully bluesy "It Hasn't Happened Yet" opts for a slyly sarcastic description of the aftermath of a romance: "You said when shadows fell/It would be hard to tell/My life from your silhouette/But it hasn't happened yet."

It may appear to some that John Hiatt is jumping on someone else's bandwagon, but either of his recent albums, "Slug Line" or "Two Bit Monsters," offers ample proof of his originality and ingenuity. And, while Elvis Costello seems to be in a holding pattern, we can delight in the eccentricities of "Taking Liberties" and hope that some of his fans discover John Hiatt while they are waiting for Elvis' next move.