I personally prefer Lightwave. The interface is a lot cleaner and easier to learn than most 3d apps, including the free ones, and the modeler is currently separate from the rendering/animation engine (although that should end in the next version or 2). I've tried quite a number of them, but I always return to Lightwave. If all you want to do is learn modeling, go with the free apps for now. The learning curve from one 3d app to another isn't that great, so if you happen to be good at modeling and eventually land a job doing it, you'll see a whole new world open up
And all that aside, if you can afford it, the professional 3d modeling programs (Lightwave, Maya3d, 3d Max, and a few others) are well worth the investment, and will afford you more creativity than you ever dreamed possible.
But also remember modeling isn't about the 3d app. It's also about the textures. You'll need to learn a graphics program like Photoshop as well to get the most out of the 3d application.
