The downside of staying at Villa is that it's just another half season not proving anything. We all know he can score goals at that level, if he scores 1, 10 or 20 for Villa between now and May then absolutely nothing changes in terms of his status or value.
At some point he has to prove he can play in the Premier League and his parent club are extremely unlikely to give him that opportunity any time soon. With us at least it's a team that are in no danger whatsoever of relegation, play good football and create quite a bit. If he hangs on at Villa then wants a loan move next season, he could end up with some right shitmunchers who never give him anything to work with and play for 0-0s constantly as they're only after one thing, safety.