Theres allot of differences between the two radios. The X has 2.4 system built into it,some more features that help with nitro,And it is fast as hell! You can only use Airtronics receivers with the X too.
With the regular M11 you can use airtronics or JR receivers. I use the regular M11 myself and love it. But I wouldn't mind the X either but it is spendy!