I went on four gorilla treks and on average was with only one other tourist. In Rwanda, you'll usually be in a group of eight. This is significant as great viewing angles can be in short supply.
Other reasons to go:
-You get a behind the scenes view of the park - I got to see a demo of their ivory/poacher sniffing dogs, got to see the gorilla rehab center (and to meet Andre Baume, who was featured in the movie!), and lots of interaction with the rangers
-The gorilla permit fee is $400 instead of $750 in Rwanda ($600 in Uganda, admittedly this value is diminished if you stay in the expensive park lodging, whereas in Rwanda cheap places are possible)
-Trekking Mount Nyiragongo (active volcano) is really, really cool
-As soon as you cross the border, you are constantly in the hands of Virunga vehicles/lodging/guards (a Virunga car picks you up at the border post and an armed guard is inside)