Go with both and build in drawers and perhaps a shelf or two. AND, you can put a tool tray under the bench top IF you leave enough room. More enclosed storage space along with more weight certainly can't hurt. The internal parts will also firm up the base quite a bit.
I'm about 6 months - elapsed time - ahead of you on building a real work bench - finally. You are going to go with through mortise and tenon joints on the stretchers/legs connections right?
You're going to enjoy making this bench I'm sure. Most of it going to be jummy wood?
charlie b