There were a few reasons. The tone being the primary one. I liked it better than the Tonehammer's. It to me was warmer, and the grit it puts out more natural sounding than the TH. It also came closer to getting the tone I was hearing inside my head. The 2 separate low and high mid semi-parametric is another. The Tonehammer only has one sweepable mid control. The HPF of the D800 + played a factor, as did the voicing control. Though it only played a very minimal role, I also wound up liking that the Mesa is located, and made here in the U.S. Overall, and this is just my opinion, you get more bang for the buck out of the Mesa's than you do with the Tonehammer.
I can't speak for the Tonehammers DI, but the one on the D800+ is top notch and definitely studio grade quality. Didn't know that though until after I got it home, and put it to work tracking. Her pre-eq DI signal is crystal clear, very well balanced.