We just returned from a wonderful safari which included 4 days of gorilla trekking. My husband and I do a lot of wildlife photography, and our vacations are planned around this activity. We have been to Africa on several occasions and this time decided we wanted to visit the gorillas. We planned our trip through Churchill Safaris, based on good recommendations on these forums and we were very pleased. Ether, the owner, was great to work with and even made some last minute changes to our itinerary. Our driver/guide Emmy was wonderful - very personable, knowledgeable, and a safe driver. We would use Churchill Safaris again in a heartbeat.
We flew into Kigali, did 2 days of gorilla trekking in Bwindi (Uganda) and 2 days of trekking at Volcanoes (Rwanda), driving between the two. We loved both experiences. I would say generally that the hiking (and viewing location) is much steeper in Uganda with more dense vegetation, however we had very good viewing on both of our treks as the gorillas moved into clearings for us, and on one day they spent a lot of time feeding on fruits in a tree. We did talk to a couple of people who were not able to see as many gorillas as our group due to the fact that they were in more dense vegetation for the entire 1 hour viewing window, so if you select Bwindi I would definitely recommend doing more than 1 trek to increase your odds of good viewing. The terrain at Rwanda generally seems more open and gentle - but there are also more difficult hikes available - again we had great viewing of the gorillas in the clearings. We did see more small baby gorillas in Rwanda as compared to Uganda, but that could have been luck of the draw, I don't know.
For overall ambiance, we preferred Bwindi because fewer people seemed to visit, the park is beautiful, and the lodge we selected - Gorilla Forest Camp - felt more remote. Some reviews have said that the rooms at Gorilla Forest Camp are "too dark," but we did not find this to be the case - the lodge is a beautifully laid-out tented camp with a wonderful outdoor eating area and the most welcoming staff. Rwanda caters to larger groups and more tourists likely because of ease of access from Kigali, and the overall experience thus feels different to Bwindi. But, you can't go wrong at either location - the gorilla experience is just fabulous.