I hate going against the grain in these type threads but I have kept several scopas and several purple tangs (I believe yellow tangs fall into this category as well) and once established I found very little difference between the two. I have had murderous purples and placid scopas but have also had the reverse be true.
I personally think a 90 is right on the cusp of keeping a purple long term, it could go either way. If the purple is the only tang in the tank and the dominant fish, I believe your chances for success will increase.
All that being said, I do think a kole tang or a tomini is almost a sure thing when it comes to not presenting aggression issues down the road. Not to mention those guys are workhorses when it comes to controlling algae. All in all though, this is just one guys opinion and I would suggest listening to many before making your decision.