I actually own a softball glove and a baseball glove. The baseball glove is a 12" Wilson and pretty ancient (dates back to junior high years). I bought a 14" Mizuno two years ago for softball specifically. I play outfield and first base so I really needed a bigger glove for softball. The Wilson (paid around $50 a long time ago) has stood up to time well and is in great shape (no cracks in the leather and has that nice, soft worn in feeling) but I rarely use it these days, other than as a spare glove for throwing the ball around. The Mizuno is a nice glove as well (paid around $75) but it probably needs to get more use than I give it to really get that comfortable feeling that the Wilson has. I actually treated the Wilson years ago with neatsfoot oil and then slept on it (under my pillow) for a week or two. I didn't do anything special to the Mizuno other than put a softball in it and wrap a few rubber bands tightly around it, but it was a nice soft leather to begin with.
EDIT: As for glove size, if you're a big guy, you might want to go with 12.5-13" glove if you're using it for both softball and baseball. I'm an average sized guy (5'10)and I find the 12" glove to be a little bit too small for softball. Then again, as I said, I play 1st base & OF where you would probably use a bigger glove.