I say if you have riden a KLX and like it - go for it. Being a KLX300 owner, I may be a bit biased however...
In my view there is only one reason to choose the TT-R or the DRZ - THE BUTTON. Other than that the KLX is a much lighter bike and has huge hop-up potential. Also it has 50cc advantage and is the only water-cooled bike of the bunch.
There is a large KLX following and a dedicated site at-
http://planetklx.dirtrider.net
The XR250 has been around forever and from what I understand is rock-solid when it comes to reliability. However when comparing the KLX to an XR, motorcycle.com chose to use the XR400. See the comparison here:
http://www.motorcycle.com/mo/mcdirt/klxvsxr.html
This is a 97 review and both bikes are largely unchanged since then.
I have to disagree with Mathew on one thing - the KLX is no MX bike. It is EXCELLENT as a trail/woods bike.
I'm sure you couldn't go wrong with any of the other bikes but the KLX was my choice and I am very happy with it.
------------------
JimmyD2
'00 KLX300R
Different Strokes for different folks...
------------------