Get the M8. M1000 will wear you out alot more......mostly from having to hang on twice as hard!!
I've rode both and the 8 was a touch easier to throw up on it's side, but it was a 153 and my M1, 162 didn't have a handlebar riser. That said, the 8 was easier to ride because you didn't have to hang on as tight, seriously.
So it comes down can you hang onto a bunch more hp for less $$ or do you want the sled that won't pull your arms off?
I'd opt for the 1000.
Min'es got a slp pipe, cpc head, intake and fuel box and the power is niiiice. About 900 mi, on second belt about 200mi ago. Hasn't blown a belt, runs great, easy to pull weight off of. The M8 felt just as heavy to dig out when it was stuck! No reason to think it won't be a reliable sled for quite a few more miles.
a 153 is what made it easier, I can tell the diff there but not in weight.