Edit: My heavily downvoted comment got deleted so I'll just repost, feel free to downvote all over again. FYI, I am open minded to new ideas, anyone at all willing to discuss where you disagree?
Ozil's accomplished more but if there was a chance to sign both for United I'd honestly take Sterling over him.
He's not that much worse technically - passing completion rate of 81%, Ozil's is 86% despite Sterling playing winger/striker and on the counter more often. Similar stats for 1st touch errors (2.3 per game vs Ozil's 1.7 per game) and number of times dispossessed (1.6 per game vs 2 per game). - (whoscored)
Sterling has decent if not Mesut-esque vision. According to Squawka Sterling's created 62 in 2014/15 at a rate of a chance every 40.5 mins, Ozil has created 39 in 2014/15 at a rate of a chance every 31.8 mins. Sterling averages 2.1 key passes per match, Ozil averages 2.5 key passes per match. (whoscored)
Not exactly a light and day difference in vision. Fairly similar stats.
Sterling has significantly better acceleration and a better top speed. He's a more direct and effective dribbler. Sterling averages 3.1 dribbles per game and gets fouled 2.6 times per game. Ozil dribbles 2.4 times per game and is only fouled 1.1 times per game. (whoscored)
They're both wank at finishing but I do think Sterling has more potential in a striking role.
Sterling's significantly better at pressing but Ozil has had some unwarranted criticism in this aspect of play.
Ozil has better set piece ability but most decent teams are already stacked with better set piece specialists, if you're an elite team looking to buy you wouldn't really care about either player's set piece ability because you already have players better at it.
Sterling's more consistent. The only criticism of his consistency has been him not being able to finish but in any team where he's not needed as a finisher he'd be great. He's a persistent dibbling threat and links up well in his build up.
His directness of play tips it in Raheem's favour. Take United for instance, Mata already does everything Ozil does but like Ozil he needs the whole team around him to be full of movement and players making intelligent runs to show off his skills. In a static team, Ozil is neutered. Sterling has that x factor where he can make things happen all on his own regardless of how sluggish his team are.
Plus he's much younger than Ozil. I say all this as a United fan, Sterling's the only one of those wankers I'd like to see play for United. Loved Ozil at Madrid but it's not as cut and dry as you're suggesting. They both have different strengths and weaknesses.