it all depends on what you want to do best... if you want to go fast in a straight line and smoke the tires off a trx 450r get a banshee. the 450r is fast, way faster than a 400ex. i had an 05 R with the HRC hop up kit, and a slip on exhaust with open air box. the banshee had K&N filter full fmf pipes, off the line the banshee would get the jump and stay about 1-2 quad lengths until i reached top speed. banshees slide around corners better. i would definately buy a 450r over a banshee if it was my main wheeler. however if i had a 400ex i would get a banshee because 2 strokes are NASTY!
beware!!! once you ride a 450r/banshee your 400ex will feel really slow. i know from experience