What is a good basic program for parametric surface modeling like Alias?

I’m quite proficient using Autodesk Inventor and have attempted to use the freeform modeling. It works pretty well but the results are completely unconstrained. I need something I can model simple car bodies and aircraft fuselages with that I can constrain accurately. I’ve looked into Autodesk Alias and it looks like it would do the job. But before I drop loads of cash and spend hours learning how to use it, I would like to find out how others are doing it. I’m not interested in figurines and statues. But I do need to do somewhat artistic shapes in which portions must be very accurately constrained.

I will be using this more as a hobbyist so cash is somewhat limited. It does not need to be full featured.