Chris,
Your PTC (or VAR) sales rep should be able to get you data that you are looking for.
I've heard and have experiences similar to what Steve V. outlined, both as a consultant and an admin.
Another source that I find very useful is TEC (http://www.technologyevaluation.com/). With their site you can go through a questionnaire and place value on functionalities and then it generates comparison reports on how each solution addresses your needs. And, though at some level it's generic answers, they do try pretty hard to get beyond the marketing hype ad actually give points on more than what the sales guys say the software can do.
~Dan