Bono's voice is richer on the Boston DVD. However, he has more passion on the Slane DVD (there's this incredibly wail right as "Streets" start), even though his voice is more "airy" (due to the ravages of touring and possibly the loss of his father).
I found the Boston DVD far more professionally filmed. The Slane DVD has more of a "home-made" look to it. This is not to say it's poor in quality - just not as polished (which is logical as I do not believe U2 were initially going to officially release this concert).
The audience is vastly superior on Slane and the energy level is also higher. Unfortunately for U2, I think the third and fourth Boston shows were better than the ones they filmed. The latter shows had an even better set-list and excellent audience reactions. As it stands, the Slane show has a better audience.
The extras on Boston are better (lots of Easter eggs), as the Slane DVD simply contains "Mysterious Ways" (which is very good and should have been left on the DVD as part of the setlist) and the previously released "Making of UF".
Overall, if you are relatively new to U2 and/or want to get into the "passionate" side of them, I'd say Slane is better. If you are more interested in seeing higher quality filmed U2, with a vocally richer Bono, and a more diverse setlist, then the Boston DVD is for you.