What's the best neu on 6s in an e-revo ?

[suicideneil]

No and no.

A lower kv motor will be slower on any given voltage compared to a higher kv motor;

kv = rpms per volt.

I suggest you have a read of my Erevo FAQ and tutorials over at the idiot forum to refresh yourself with wonderful BL knowledge, then the answers to your questions will make more sense hopefully:

http://monster.traxxas.com/showthread.php?t=430233

As has been asked, what exactly are your intentions or desires with regards to your truck- speed machine, racer, basher?...