Funny.... all this advice and this is the only post that addresses one of the most important deciding factors: terrain. Open desert... yes, a 450 is great, but have you ridden such a pig on anything slightly technical? My gawd it's awful. There seems to be a lot of blind advice here.
Do you like wringing out a bike for power, or do you enjoy lugging them? Riding style is pretty important as well. If you're less-aggressive, or tend to get yourself into trouble A LOT (like some people I know), perhaps a smaller displacement bike is better for you. Sit on the bikes: specifications LIE. See what "feels" lighter to you... see what ergos you like.
If I had a dollar for everytime I saw a guy on a 450 in technical terrain, overheating, pissing coolant, and tipping-over every 12 feet, I'd be rich.
Think about what's most important to you, and make your decision based on that.
Don't let the tight stuff put you off the 450 as long as you're fairly fit & strong(I'm 57,not too fit), although I agree a lighter bike makes tight riding easier. Had my 450WR now going on two years & raced harescramble/enduros for those two years without any maintenance issues except for the regular oil, filters, tyres, two new plugs & only now am I replacing steering head, wheel bearings, sprockets & chain & brake pads.
Can't beat the Yammie 4T's for reliability...........thats a fact!