I don't think you are gonig to get a clear conscise answer from this forum.
It's going to involve lots of opinions and personal experience...and maybe even some brand loyalty. LOL
Honesly, in MY opinion...the chassis are quite similar.
I spent a day last year riding all of the 2010 models for the entire day. I hopped back and forth as much as I could...probably spent more time on the M.
I also did this in 2009 where both involved Sunridge Sledhead's turbo'd sleds. A 2007 turbo m7 and a 2007 turbo dragon.
On the 2007 turbo sleds, I liked the Dragon better. Much better.
On the 2010 stockers (Dragon was a 155 and M was a 163), aside from minor handlebar ergos (Protaper bars on dragon were too wide for me), I had a hard time telling the difference between those two sleds.
IMO, it really depends on what vintage M you chose.
If I was comparing your two choices as stocker sleds (the 2010 M and the 2008 Dragon), I'd get the M if it were me.
BUT factor in the power and fun of the turbo, cost of maintainence, race fuel and difference in cost of the purchase....that's a whole new ball of wax.
And you CAN put a price on a smile.....
I'd say ride them both, decide which one toots your horn, and then decide which one you want to afford.
You ride it...you decide.