Get the 450X. I have ridden both of them & your money is well spent on the bigger bike.
They both go well. But I believe you will out grow the 250 in short order.
The 450 has the ability to be ridden slowly and on trails it's fine.
When you want to wring it out, it has a decent top end.
It's not all that impressive to me, but for most it's a bike that will impress.
The 08 changes have made the X quite a more versatile & user friendly machine.
Besides, A little extra motor never hurts.
Whatever size bike you choose, Get the suspension redone for you & where you ride.
It will make a night & day difference in how that bike works.
If you wanna talk trench digging,102 mph, scare the crap out of you, stands on end in every gear bike. Get on a proper tuned XR650R. Now, thats a machine.