if you are going to race it on a track, then to me its worth it. If not, then I personally would NEVER EVER go back to a 450.
The handling is very similar for off track riding. There is a lot more power to be had with the 700, and twice the torque.
The 700 is more comfortable and has more creature comforts built in.
The 450 though, you can make into a very light/agile missle on the track.
I was a big 450 guy myself. Started off with a warrior, then banshee, then 660, then 450 than 700....gone threw em all. 450 was hands down my favorite till the 700 came out. I wasnt track riding anymore, and the 700 did more things better or on par with the 450 and it just didnt make sense to own the 450 any longer....especially when we bought the lady the first 700 as the 'girl quad'. HA!
There is no 'best' with either of em...just depends on what you are doing I suppose. I like em both. All I know is I still ride with the same 450 chums I always have and my 700 walks them pretty easy...I can woods race just as good and I am comfortable, fuel injected, and have reverse to boot.
The 04-09 standard yfz has almost identical numbers as far as width, length and seat height...only diff is the standard 450 weighs 350 dry/ 372 wet....and the 700 weighs 396 dry/422 wet. 50 lbs...which, if you are throwing the machine around big time, YOU WILL FEEL. For instance, you are in a left hand slide, and have to transition to a right hand slide...that is when you will feel it. As far as anything else, IMHO they handle very very similar.
Power is all bottom-mid stock on a 700, and all topend on a 450. HOWEVER, get your 700 breathing like mine and they have topend to boot..and will whoop up on 450's pretty bad. 450's spin...700's go. Difference in tq, rotating mass, and gearing.
If you are thinking the new 450R....well, you are going to have a machine BUILT for the track big time with its stance, but its also a lot heavier with that race equipment tipping the scales at 405 wet. But the power comes on sooner than the old 450 and you'll have a deadly weapon for the track. I personally wouldnt want a 49-50 inch quad for woods racing though.