I think all three bike weigh about the same (255-265) lbs. The DR-Z has a stronger motor than the XR but the WR runs away from both of them. The suspension on the XR and DRZ will be softer than the WR . The XR is gonna be the simplest to maintain while the DR-Z and WR are gonna be a little more maintainence intensive. Unless the WR has the YZ seat-tank combo it will be awkward to ride because of the fatty gas tank on early WRs. The XR 400r will be more comfortable to ride on most trails because of the size and shape of the seat but will be lacking in power when compared to the other two (especially the WR). The DR-Z and WRs are really tall (38-40 inch seat height) so if you are shorter than 5'9" you may be happyier on the XR (36.7 inch).
A properly jetted XR will start in 1-3 kicks, a WR will also start in 1-3 kicks depending on whether you get the manual decomp starting procedure down. The DR-Z lights at the push of a button
(unless you get the "K" version). All three good bikes, all depends on how much you want to spend