450 Differences
 

They are both competition bikes, one is an enduro bike the other is a motocross bike. Neither would be suitable for any long distance trip. I believe the WR450F would need valves checked at 5000 and the MXer wouldn't last more than 30 hours. Forgive me if I'm wrong as I'm just going off the top of my head.

Actually, among the new generation 4 strokes out there, Yamaha by far have the most reliable and proven bike. It requires the least maintenance and has a stellar reputation in this world. But that world is a different one from going RTW.

The Yam about beats them all: This would include
the Honda CRF450X, Kawi KX450, Suzuki RMZ 450, Husky 450, Husaberg 450, KTM 450.

Actually, the new Kawasaki 450 enduro bike would no doubt match the Yam WR450. Still, not a tourer by any stretch.

The WR's are going years with just routine maintenance. The MX version is truly a race bike .... not a good choice for long distance. But even the MXers are very reliable in that class.

Biggest problem I see with taking the WR450 would be carrying luggage.
The subframe is not designed for a big load. If you could keep you load down to maybe 60 lbs. you might be OK. Or beef it up some. Acerbis make BIG tanks and they get fantastic mileage if you don't get on it. My little WR250 get about 60 mpg if I"m just plodding along.

I'm a big fan of the WR, but as Mollydog said, not for RTW. Having said that there probably isn't any bike that is incapable of doing a RTW, but that statement also needs to be read in the same vein as there isn't a bike that is incapable of doing 50 odd laps of a Grand Prix circuit. When it comes to suitability of the bike, the simpler, more agricultural with higher tolerance levels, the better. Clue that in with steel frames etc etc and you are left with a bike that on paper at least is a bit of a dog. On that note, the XT is your best friend.