Celsus preferred beef while Galen preferred pork in terms of providing the best nutrition.
A Comparison of Ancient Greek and Roman Sports Diets with Modern Day Practices
In the early days, athletes relied on their trainer to make sure that their dietary needs were met. However, it was not long before medical doctors took over, and the first sports physicians were created. In a report from Philostratos we learn .
“...The Sicilian style of fancy food gained popularity; the guts went out of athletics and, more important, trainers became too easy on their pupils. Doctors took the lead in introducing permissiveness, setting it up as an adjunct to their treatment...from these Doctors athletes learned to be lazy and to exercise after sitting around stuffed with enough food to fill an Egyptian or African meal sack; they gave us chefs and cooks to please our palates. They turned athletes into gluttons with bottomless stomachs.”
However, whilst it was popular or fashionable to have a physician designing your diet, it seems that these medical doctors did not always share the same opinion about what the athletes should eat. Celsus, who was not trained as a medical practitioner, although he wrote a great deal about medical practices, and Galen, who was medically trained, did not agree on the type of meat that was the “strongest”, that is to say the most nutritious, for an athlete. Celsus preferred beef whilst Galen, who was particularly enthusiastic about the advice given, considering him to be an expert on diet and exercise , gave the Olympic gold, so to speak, to pork, which he felt was the most nutritious form of meat. Maybe this was based on his own positive experience with pork when he was a medical practitioner in Pergamon where he took part in the training of gladiators.
Ancient athletes would most likely not have been able to afford very much protein in the form of meat, and would as a consequence not have eaten meat on a daily basis. However, we know from Celsus  and Galen  that meat in the form of terrestrial and aquatic livestock was considered nutritious, and was classified among the “strong” foodstuffs. Celsus and Galen [19,22] could not, however, agree as to which meat was the “strongest”, Celsus  favoured beef, whilst Galen  never misses a chance to sing the praises of pork, which alongside fresh milk was his favourite food.
“Among food from domesticated quadrupeds pork is the weakest, beef the strongest. And so also of game, the larger the animal the stronger the food yields” 
“Flesh, when well concocted, produces the best blood, especially in the case of animals such as the pig family, which produce healthy humour. Pork is the most nutritious of all foods, and athletes provide a very visible test of this. For when, after identical exercises, they take the same amount of a different food on one day, straightway on the following day they appear not only weaker but also obviously less well fed.”
“Beef itself gives a nutriment that is neither small in quantity nor easily dispersed; yet it produces blood that is inappropriately thick” .
“Lambs also have flesh that is very moist and productive of mucus. But that of adult sheep is more productive of residues and more unwholesome. The flesh of goats is unwholesome too, with bitterness” .
Poultry was also considered a nutritious foodstuff, although here size mattered. Celsus  ascribed poultry to the “medium” class of foodstuffs, whilst Galen was not so generous in his appraisal, preferring once again to extol the virtues of pigs and terming poultry meat as “poorly nutritious”.
“Likewise of those birds, which belong to the middle class, those which rely more on their feet are stronger food than those which rely more on their wings; and of those birds which depend on flight, the larger birds yield stronger food than the smaller, such as fig-eater and thrush. And those also which pass their time in the water yield a weaker food than those which have no knowledge of swimming” .
“The family of all winged animals is poorly nutritious when compared with that of terrestrial animals, especially pigs: you would find no flesh more nutritious than theirs” .
Fish too were classified as a “middle” foodstuff by Celsus , although here preference was given to the oily fish such as mackerel in comparison with bass and mullet. This is in accordance to general recommendations today concerning intake of oily fish like salmon and tuna, although the reason given here is to prevent heart diseases. Galen goes one step further in his assessment of fish, telling us that they are not appropriate for athletes but should rather be reserved for those who are weak and ill.
“The fish most in use belong to the middle class; the strongest are, however, those from which salted preparations can be made, such as the mackerel; next come those which, although more tender, are nevertheless firm, such as the gilthead, gurnard, sea bream, eye fish, then the flat fish, and after these still softer, the bass and mullets, and after these all rock fish” .
“But from all the above fish the nutriment is best for those who are not in training, and the idle, frail and convalescent. People in training need more nutritious food, about which there has been previous comment” .
“...the best milk is just about the most wholesome of any of the foods we consume”.
“For cows´ milk is very thick and fatty, while milk from the camel is very liquid and much less fatty; and next to the latter animal is that from mares, and following this, ass´s milk. Goat´s milk is well proportioned in its composition, but ewe´s milk is thicker” .
“Its continued use also harms the teeth, together with the flesh surrounding them, which they call “gums”. For it makes these flabby, and makes the teeth liable to decay and easily eaten away. Accordingly one should rinse the mouth with diluted wine after consuming milk, and it is better if you put honey with it” .
“Moreover it is neither unwholesome nor very markedly productive of thick humour, a common charge against all cheeses. A very fine cheese is the one highly regarded by the wealthy in Rome (its name is bathysikos), as well as some others in other regions” .
“Among pulses, beans and lentils are stronger food than peas” .
“However, they [Figs] do not produce firm, strong flesh like bread and pork do, but a spongy flesh, as the broad bean does” .