I would agree that loud & fast is in the MBF's wheelhouse; it loves classic rock and I love it for that stuff.
I'm also happy w/it so far for URB, but as I mentioned earlier, my experience is limited to direct in at fairly low volume so far. I've yet to try using the A & B channels for doubling EB + URB, for instance, though I have a plan & it works out great at home. I just don't have the right gig for it any time soon.
I also can't help w/your question about about the SL, since I've never tried one. I haven't seen much feedback about it for URB though... maybe I'm not watching close enough. God knows there are enough comments on TB

.
I did use a Walkabout for a busy weekend years ago. It's a wonderful, tube-ampish tone, no question, but I couuldn't make happy with the control scheme on a couple big stages in a row, so that was it for me... probably too rash, but when you're biusy... I know many folks who love 'em (plenty right here on TB) but most I know use 'em for EB, not URB, so I'd rely on Chef for that particular assessment.
I do play with a WW once a month (EB) and can say that (at the risk of many flames I'm sure) for EB at least the MBF is closer soundwise IMO to the WW than the Walkabout as I remember it. I'm sorry to say that I've never gotten to use a WW for URB giggage, where many of its biggest fans reside.
Good luck with your choice!