I own both. They are totally and tonally different animals as has been noted here.
The M6 Combo slightly pushes the limit on "combo" criteria. It's rather easy to move around...until you need to negotiate stairs or tight spaces. It's very bright, clean, punchy, and deep.
The WA 12 Combo alone is a freak of nature. Something that little should not sound so big, but it does. However, it is only a single 12 and has it's limits. I've pushed it to it's limits and it hasn't let me down yet. I prefer the tone of the WA so I added a 12" extension cab. I run it at 2 ohms with no problems. In this configuration, it can get very loud, deep and punchy and is somewhere in the neighborhood of 500 watts.
The M6 is just a beast, and to me, it feels like it could make the 212 cab cry for mercy before it even got close to running out of steam. I've used the head with other cabinets and it has always sounded great. I use the combo when I need more horsepower.
My first grab is usually the WA plus extension cab. Very modular, plenty of volume for most gigs, and I absolutely love the EQ. I've tried the WA head with the 212 combo cab and it sounded wonderful. Plenty of usuable volume, but not really close to the headroom.
Sorry for the long post, but the I think the OP made the right decision for his gig situation.
The M6 really won't do the darker, tubier give thing that the WA can do. The WA can get closer to the M6 tone than the M6 can get to the WA if that makes any sense. Of course, the M6 will spank it in volume and headroom.
All in IMO...
