The banshee is alot faster stock for stock, but it also takes twice as much money to modify a banshee since it has 2 cylenders. Also a maintained 250r is more reliable than a banshee (or any yamaha). If I had to choose between the 2 I would take a 250r any day, but thats because it is best for my riding style ( mx, trails, gravel pits, dunes once a year).
Thats just my opinion, im not flaming the banshee

 

In my experience i have never seen a 250r beat a banshee...most of the banshees around here are moded but just mostly bolt on stuff and no rs usually touch them.There was this one 250r though that was all decked out and had a 330 kit but the only banshee it raced was my neighbors very hopped up one running on 108 race gas,etc..and it got beat pretty bad!Not to say a 250r cant beat one,just it takes a lot more to get it up to the level!