The best buses to use from Kampala to Kigali include; Trinity Express, Modern Coast, Volcano, Kampala Coach and Mash Poa. They have a fleet of new Yutong and Scania coaches which are quite comfortable and all windows do open if you need some extra air. Buses are equipped with a TV so you will have your fair share of mostly gospel movies and music videos. As usual the buses don’t have a toilet but they do make enough stops along the way so that you can survive and of course there is a longer stop at the border. Remember that Rwandans don’t like plastic bags! Trip takes around 9 hours but there is some interesting scenery along the way + crossing the equator near Kampala.
Another good company that people often use is Jaguar Coaches. Their bus terminal in Kampala is right next to Trinity one, so you can easily compare both of them. Both have similar schedules with day buses leaving between 7 and 9AM and then there is also the night bus. Prices are the same, 40000 shillings. In Kigali both of them use the Nyabugogo bus terminal.