Welbeck will have much more consistent game time in his natural position at Arsenal than at United. Welbeck will be ahead of all their strikers and as seen against Everton, Sanchez seems uncapable of being as influential as a striker than he is as a wide attacker or second striker.
Nevertheless, I do not believe Manchester United will be that stupid to sell Welbeck to Arsenal or even a top 6 club with the way we have been playing. He would greatly improve the Arsenal team and become a sure fix in their attacking department. He is the type of striker Wenger has wet dreams about to develop in the vision of Bergkamp, Henry, Adebayor, etc. If Welbeck wants a big contract, this is perhaps the best time to get it at United because his impact at one of those club could mean another season without champion league football at United.