I have the 12", got to try the 15" version before I bought and decided that the 12" was the best way to go for me.
Reasons:
The 12 was full AND punchy while being reasonably sized and portable.
The 15 was significantly larger/bulkier and so less portable, it sounded less focused/punchy (to me anyway) with a tendency to get flubby in the lows without some significant attention to the EQ, which I'd rather not have to mess with so much.
I feel like this is somewhat due to the passive radiator design of the WA. The 12" ver. has either a 8" (older version) or a 10" passive radiator. The 15" ver. has either a 10" (older) or a 12" passive radiator design. Its possible that Mesa had a harder time balancing out the passive radiator factor on the larger cone of the 15 system. Not really sure and no science to back this up, just a hunch.
So, there are my reasons to go with the 12.
That being said, I did eventually pick up a Scout 15 to run as an extension to my WA 12 combo. I really like the combination when I use it .... Go figure!
good luck