There are many demos to try, and I would suggest using the demos before you buy.  Lots of programs look great, but end up frustrating you with counter-intuitive workspaces.  There's nothing worse than knowing exactly HOW to model something, but find you can't because you either can't find the tool, or can't move the view around efficiently enough to do it!  So try any software you're thinking about buying first before making a decision.

I got very good deals on Hexagon 2 (through the Platinum club on DAZ), Shade 8 (through Content Paradise), but I find myself liking other packages better because I can actually move the window around without a lot of headaches.  So I now have what would be considered a collection of very expensive programs (had I not gotten the deals or upgrades) that I don't like to use.

I would think of a very basic modeling excersize, like making a pair of dice, and try to do it in each of the demos.  Note how long it takes you to find and use the tools needed to do this in each package.  That might give you an idea if a particular piece of software would be useful to you.

Also consider how much helpful support you'd get for each program you consider.  The Cinema 4D forum here is very useful, but Shade's section doesn't have a lot of traffic.  And Hexagon doesn't even have one!  It always helps to get started in a new program if you have somewhere to go to ask all those fiddly little questions.

And for the record, I would advise against Carrera.  It's been years since I've used it, but even back then it was used more for rendering scenes created in other modeling programs.  I wouldn't consider it a useful modeling tool.