Just got springfield downs and done some research.
I think the favourite is the best to bet on every eight hours. All the dogs have the same return in money over enough time (the odds are all in your favour as the payout is much than the chance) but the favourite has a steady stream, and also better XP return.
I guess if you're at the later levels and money is no problem, it'd be more fun to go for the jackpot, but the odds are stacked in your favour no matter which you go for and it's not really gambling (unless it changes later on when you get fat tony lol). The only difference is XP, assuming 50 xp is the standard for all losers, in 66 bets for the 99/1, you'd only get 3750 xp, but in 66 bets on the favourite you'd get 4125 xp