just depends on riding style.
sand dunes=450
drag racing=450
trails=400
nice easy goin wheelies=400

the power of the 450 is violent compared to the 400. my buddy has a 06 450 and i would take my ex over it any day. reliability of 400 is much better. then again water cooled. yah know its all prefferance. i personaly like my ex its funner to ride. why not upgrade to an 05 ex or newer so u got reverse. i will never give that up. good luck in ur decision. nothin beats bringin a new bike home to drool over though.