As far as difference in dbs go i would imagine something like 5-8db...depending on how well the box is tuned if its a SPL box.
Which would make a difference to how loud it goes. As stated above, it'll only sound good with a handful of songs.
Id say if your not after a system that you wanna compete (at SPL comps) with and are concerned about box volume / size, go with the sealed version. It wont be as loud as a ported box but will sound good with all music and be relatively small / light.
A sealed box, provided its the correct volume, will also give you a flat frequency response and therefore decent sound quality.