Who should be the horse of the year -- John Henry or Slew o' Gold? The choice for the Eclipse Award has never been tougher. Slew o' Gold scored five easy victories in a row before losing a photo-finish decision in the Breeders' Cup Classic. All his races were on the dirt. John Henry was six for nine, finishing his season by winning four straight major stakes. All his victories came on the grass.

Slew o' Gold earned more money than any thoroughbred has won in a single season -- $2.6 million. John Henry won $2.3 million, the second-highest total in history.

Not only were they productive, but both horses were, in their own way, admirable. John Henry defied the laws of geriatrics to accomplish so much at the age of 9; he still is a fierce competitor. Slew o' Gold managed to excel although beset by foot problems all year. If he hadn't cracked his hoof for the third time just before the Breeders' Cup, he probably would have been undefeated in 1984.

My own racing prejudices make me want to vote against John Henry because he is a grass horse. The ultimate test of U.S. racehorses is running classic distances on the dirt. I don't like the Europeanization of our racing; I thought it was a travesty when All Along was voted the horse of the year in 1983 for winning three races on European-type soft turf courses. Slew o' Gold deserved the title far more than she did.

But Slew o' Gold's record this season isn't nearly as impressive as it looks. He was able to sweep New York's fall championship series because the opposition was so mediocre, notably Shifty Sheik, Bounding Basque and Canadian Factor.

John Henry wasn't winning his races by default. In the Budweiser Arlington Million, he beat Royal Heroine, who subsequently set a world record in the Breeders' Cup Mile. In the Turf Classic at Belmont, he whipped All Along and the rest of an excellent field. Because he beat formidable opposition, while Slew o' Gold didn't, John Henry deserves to be the horse of the year.

The rest of the Eclipse Award choices are relatively easy, largely because the Breeders' Cup provided definitive championship tests in most categories. Still, some voters may go astray for reasons of sentiment, because principal horses in three divisons died or almost died during the season.

TWO-YEAR-OLD COLT: Chief's Crown may be the weakest juvenile champion in more than a decade, but he is the champion, nevertheless. His victory in the Breeders' Cup should have clinched the title for him, although some sentimentalists still may vote for the undefeated but overrated Saratoga Six, whose career was cut short by an injury.

TWO-YEAR-OLD FILLY: Although Outstandingly won the $1 million Breeders' Cup race and the $500,000 Starlet Stakes at Hollywood Park, she did so in such abysmally slow time that I'd have to burn my speed figures before I could vote for her. Folk Art, a filly who won three races in California before a minor illness sidelined her, gets my vote. The 1985 racing season will prove me right.

THREE-YEAR OLD COLT: What do you say about a 3-year-old colt who died? That he was grossly overrated? That he won the Kentucky Derby and the Belmont Stakes because he had perfect racing luck? That's what I say about Swale. Gate Dancer was the best 3-year-old of the year, as he proved with a smashing victory in the Super Derby at Louisiana Downs and his near-miss against older horses in the Breeders' Cup Classic.

THREE-YEAR-OLD FILLY: Life's Magic was the best of this uninspiring group.

OLDER HORSE OR GELDING: Slew o' Gold is the obvious winner in this category, which implicitly is limited to horses who race on dirt.

OLDER FILLY OR MARE: Princess Rooney was the most dominant member of any of the Eclipse Award categories. Her runaway victory was the most impressive of all the performances on Breeders' Cup day. She may even get some votes for horse of the year, but because she never even attempted to race against males, she doesn't merit that title. If she had been given the chance to face Slew o' Gold, though, she might well have beaten him.

MALE TURF HORSE: John Henry, of course.

FEMALE TURF HORSE: Royal Heroine could beat any member of her sex, and almost any male but John Henry.

SPRINTER: Eillo clinched the title with his victory in the Breeders' Cup Sprint. His award will be posthumous because he died earlier this month after an attack of colic.

JOCKEY: Because he rides principally in the Midwest, Pat Day doesn't win the big money that Angel Cordero Jr. and Laffit Pincay Jr. do, and so he usually gets overlooked in the Eclipse balloting. But when riding head to head against the big names of his profession, he often outperforms them, as he did when he rode Wild Again to his upset victory in the Breeders' Cup Classic.

OWNER AND TRAINER: Until somebody proves for certain that Oscar Barrera is using the most potent drug in the history of pharmacology, his accomplishments deserve to be taken at face value. Few horsemen in history have done what he did with Shifty Sheik, claiming him for $35,000 and almost beating Slew o' Gold a month later. He deserves two Eclipse awards.