I tried them both out at Summer NAMM this year. I didn't see the A51TT, but the "Commemorative Edition" of the A51TT (called the A51CE) was set up along with the A48 and a few others. I remember liking the A51CE much better on my voice than the A48, but I don't remember why. The CE has a different case and tube than the 'TT, but supposedly, you can replace the tube in the 'TT and get real close to the 'CE's sound.
The A48 was no slouch, though, and I'm sure it would work great on lots of stuff!