If you're going to be making precisely measured parts I would suggest Sketchup, and I'd then also suggest investigating some various add-ins which add more functionality. Sketchup is suited more towards architectural modeling by default, but there are various free add-ins/ruby scripts which give capabilities such as boolean operations and easy fillets, which make solid modeling easier.
I actually use SolidWorks mostly, but have been interested in checking out this new program from Cubify which looks like it works very similarly and is intended to be used for 3D printing design.
http://cubify.com/products/cubify_invent/index.aspx
It's got a free 15 day trial and is only $50 past that.
The main practical differences related to modeling for 3D printing that I've seen between free programs like Sketchup and expensive professional programs like SolidWorks is sometimes you have to go through more steps in the free program to get the same ends.
Last edited: Aug 27, 2012