Frankly speaking, I dont think he will be good at Chelsea. His ball control is non-existent and his finishing isnt the greatest either. As for the important goal or assist, he hasnt done that consistently enough for me to want Arsenal to put an extra effort to keep him. I can remember Bendtner popping up with some important goals and assists in one of the seasons... that shouldnt be a reason for him to be at the club.
I think Sterling and him are on the save level but Sterling is younger, so there is room for improvement. Theo is near the end of his 'best career phase' and he also had an ACL surgery, which is one of the worst surgeries to have for a quick player.