There's a lot of hype surrounding PowerScore and TestMasters, in large part due to these forums. When you look at the course offerings side by side, there are a few but important differences between them and the only other course I would consider, Princeton Review (Kaplan is always getting trashed in these forums so I never considered them). Dig past the hype and look closely at the nitty gritty details for each company. Call and ask them very specific questions.
I researched this a while ago and don't remember all the details, but here's what I do recall:
TestMasters: They have a compelling course offering but it's not the perfect solution that many people tout it to be. They talk a lot about their instructors' 99th percentile scores (nor as much on their instructors' teaching abilities). Also can have huge classes depending on where you live and don't give printed explanations, so if you need help you need to get it from an instructor, which is tough if you have a large class because there's only a half hour at the beginning for questions. Once you receive your course materials, you can't get a refund, even if the course hasn't started. So once I looked past the "TM is the best!!" mantra I found a lot of chinks in the armor.
PowerScore: Mainly well-known for the Logic Games Bible (definitely a great book), but I don't know much about their courses other than the fact they're mostly a TM clone. They don't have courses near me so I didn't look at them that closely.
Princeton Review: Recently revamped their course to bump up the number of hours, and added in-class workshops. They focus a lot on instructor training and have small classes. They also have a very good guarantee program. I was predisposed to PR because a friend of mine took them before they revamped the course and was very happy. When I spoke to people at PR they were also very friendly and helpful and very forthcoming with information. I got a very good feeling from them and was impressed with the improvements they've made. They are clearly aiming at competing more directly with TM. I have Cracking the LSAT and it's excellent.
All of the courses use the official LSAT tests, so that's not a factor.
I have not yet taken a course but will be taking PR's if I do take one.
Hope this helps...