400ex all the way man but im an owner. for the begginer in mind on the trails there best the 450's can be learned quickly but are a bit harder to learn on in the trails.
I would if looking at the Z400 or KFX400 or DVX400 I would get the KFX there the best over the other 2. There exactly the same all the way around but the KFX has better suspension from the factory even over the EX and is a great machine but the EX is a way reliable bike and is also several hundred cheaper.