Realistically, Tails would almost certainly win - she has far too many advantages.
One of these is that she is bottom-heavy (as advertised), making it harder to knock her off-balance and giving her a much sturdier base to launch punches (which, as all martial artists know, are powered chiefly by the hips.) Any hit to the top half of Sonic will almost certainly send her stumbling as a good third of her body weight is in her boobs.
Another is that Tails is much shorter, allowing her to remove most of the strength from all of Sonic's attacks simply by standing as close to her as possible. Sonic, on the other hand, must maintain an optimum range that is neither too close nor too far. While Sonic has a mild advantage in that her punches go down and Tails's go up, Tails is so much shorter that she may not even be able to land a clean hit on her at all without stooping (assuming kicking is not allowed.)
Oh, and there's a massive soft pair of fat tits Tails can keep brutalizing until Sonic gives in. It would in fact be far more difficult to land any other legal strike, and she's not going to come anywhere close to Sonic's head without leaping.
Realistically, Tails would almost certainly win - she has far too many advantages. One of these is
If Tails is quick on her feet and stays out of reach, she could turn those big ballons into punching bags! Go Tails! Jab, punch, Uppercut! Throwing blows up your guts!] (if you get this reference, you win a cookie)
If Tails is quick on her feet and stays out of reach, she could turn those big ballons into punching
