I have a bootcut pair and I just bought the AG Stilt skinnies that Angie has and recommends (and I love them!). I'd say that the best shape is the one that you wear the most and makes you feel happiest in. I like the bootcuts, but I love skinnies, so the AG skinnies are my favorite.
I had to size up on my AG skinnies, but not on my Gap bootcuts. However, I definitely say that you should lean toward slightly too loose versus too tight. I am on team fitted and always go for slightly tight over slightly loose on denim, other than white. Any tightness with white denim is very unflattering.