The current versions of the Rumble amps are lighter than the older versions - Patrick, maybe you had the older ones?
I haven't owned either, but I've played both the Rumble 40 and the B50C in store (with electric basses and a short scale acoustic guitar, which is pretty close to a uke in tone). And I did a workshop (on bass) with B50C's, so I had a day playing one in a group setting with others playing them too. It's hard to give feedback on tone over the internet, but they both sound fine. I didn't particularly like the overdrive built in to the B50C but most people probably wouldn't use it much anyways. I liked the EQ and the bright switch on the Fender better than the EQ on the B50C. The B50C's controls are on the front of the panel, which is a nice touch if you like to fiddle, since it's easier to see them from in front. And the kickback feature means it works great as a monitor, especially if you're seated. Ultimately I think I liked the tone of the Fender a little more - although they both sound good, but the Acoustic was a little more "practical" in it's layout/format. The workshop setting was very telling from that perspective, you had a bunch of people sitting on stools with their amps pointed right up at them and controls were within reach. Helpful when you're playing with an unfamiliar amp in an unfamiliar setting. If the workshop had been done with the Fenders, I think it would have been a little more awkward since everyone would have been leaning over all the time trying to see the controls and tweak things. But if you're just playing at home, and are going to dial in the settings and leave them, that might not matter at all.