It is a hard decision, 400ex's are one of if not the best all round built quad out... think how great a 250R is... well it's a more reliable 4-stroke 250R... but the Z-400 does have more power. It is a hard decision, I like the 400EX and the Z-400 both a lot. Personally I would get the EX, but that's only because more mods are available and I like it's stance, handling more... plus nothing will touch it's steering (all though z400 is close). The only problem is you'll need to mod out the 400EX just put out the Z's stock HP... so it's up to you. W/E you plain to do with it I guess... I will say this though, the Z is a sick trail quad...very sick!