I've played both and bought the Walkabout Scout 12" model.
The Ampeg B100 had a nice fat sound, well suited for old school bass parts. In a very small room with a not very loud band it would probably do allright.
The Scout blows the Ampeg away in volume, ability to project into a room and flexibility of sound. Lots of good features too. It has the ability to get a fat, tubelike slightly compressed sound that makes it sound much like the B100. You have to mess with the controls and the adjustable horn, but it's not all that hard. It also can get a very clean but full sound, in my opinion, better than a GK can. The Ampeg can't do that nearly as well, even at low volumes.
In my opinion, it's impossible and unfair to compare the Ampeg and Scout. Just look at the difference in price, quality of components and intended use.
I've used the Scout for over 2 years now and had no problems.
I like the sound of the Ampeg, but it is basically just one sound and has a very limited volume and application. The Walkabout Scout is in a different class. It's also smaller and lighter than the Ampeg at 50 lbs for the 12. I'm not sure what the 15 weighs but you can check the Mesa website.