I ran into this problem awhile back, and although I liked the idea of using punches, some of my curves are a little complex and I could not buy enough punches for some of the curves I was doing. I chose to search for a good, all purpose pattern cutting knife. I think a round knife is probably the best solution, but not being near any dealers (so I could try them out before investing the money, as they are somewhat of an investment) I decided to see what else would also work.
The knife I use now for cutting pattern pieces, and get fairly nice results with (i.e. nice clean-cut edges), is an X-acto #5 heavy-duty knife. (Click here to see one.) I'm still using the original #19 blade that came with it; I just keep stropping it, and it has held up quite well so far.
By the way, I'm using this mostly on 5-6 oz. leather and lighter. For the lighter weights, you may want to experiment with the small-diameter rotary cutters.
Kate