should i buy a cr250 or a xr400

For dunes, CR.

for everything- cr, except enduro, look into yz400's too

CR vs. XR Go with the CR for sure unless you will be doing slow forest trails like in Big Bear. Although if you know nothing mechanically and are lazy with maintenance get the XR cause they run for ever with no need for work. I personally agree with Taco, check out the YZ400/YZ426. I ride the YZ400 and blast around the dunes and have smoked many 250s around the track. The 250 and 400 are very similar performance wise just require a different riding style. The YZ400 will KILL a XR400.
Khris

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When in doubt gas it!!

toooo tell you the TRUTH think ahead........... what kind of riding are you planning to do--fast long deser--or more of a track bike