I'd say, if you're buying a box that's already made, a sealed box would be best. Just make sure it has enough internal volume, and is made with 3/4" MDF.
If you're going ported, you need to have one made specifically to those subs.
As for which would be better in your car, and it being a convertible, I'd have no idea. I know ported will usually tend to be louder, but also takes up twice as much space. As to it being a convertible though, I believe you could run whichever you'd like. I know a dude that has a pair of 16's ported in his trunk, convertible, **** near shakes the car apart. Mustang too. Here's the vid: Dual 16Os :: CIMG1973.mp4 video by joeybutts - Photobucket