It doesn't matter, they both provide the same service once you get tuxguitar or guitarpro or the like, the only difference is that you can download ptb and gp files and access them offline.

However since we're on UG i love UG with all my heart.
I actually prefer Songsterr, because the fact that it plays the tabs let's you know how accurate the tab is. Plus the community can edit one tab to perfect it, rather than 7 different tabs of varying quality on UG.