The 10" combo at 15 1/2" x 14" x 11 3/4" and 18lbs is about 10% smaller than the Walkabout 12", and is about 1/2 the weight
You make valid points, but I have to hold you to your own standard. Mesa doesn't publish the weight of the scout combos, but, they advertise the scout 12" cabinet as weighing 37 pounds, and the Walkabout head weighing 13 pounds. Add the two together and you get 50 pounds. The Shuttle 3.0-10t combo weighs about 18 pounds, which is closer to 1/3 the weight than 1/2 the weight. Even the Shuttle 6.0-12t combo is a lot lighter than the Mesa, at 31 or 32 pounds.
However, you do make very valid points, as I said. Just wanted to get the weights in order.
Now on to my general thoughts, with the caveat, of course, that I remind everyone to see my endorsement disclosure when reading my opinions. They are honest opinions formed well before the point at which I became a Genz Benz endorser, but still ...
The one big advantage the Shuttle 6.0 12" combo has is expandability. It's putting out its 375 watts into 8 ohms, and puts out over 600 watts into 4 ohms if you add an extension. That being said, I do recall seeing someone slave Walkabout combos. If you order them in 8 ohm configuration, you could easily throw a double-stack with 600 watts into 4 x 12, or 4 x 15. Of course, you could also slave two Shuttle 6.0 stacks, and have 1200 watts into 4 x 12, etc.
Honestly, I think it comes down to tone goals, combined with weight and expandability. If you want a combo that sounds like a good old tube amp, with all the (imho) great tones that provides, and you don't need expandability so much (or don't mind slaving multiple combos), I don't think you can beat the Walkbaout Scout. Disclaimer here is that I've only played with one for a little while in a store, but these were my impressions, and they seem to mesh well with what owners report.
If you want a cleaner tone, but with some tube warmth still available, and like lighter weight and expandability, a Shuttle 6.0-based combo is killer.
All of the above is very much imho. And I hope it comes through that I really like and respect Mesa gear, regardless of the choices I have made for my own gear.
Respectfully,
Liam