Hi Debbie
It really depends on what you want to get out of going to classes. Any instructor worth their salt will offer lower impact/intensity options during a regular aerobics class, so it doesn't have to be a high impact jumping-jack-fest. Aqua is non-impact by its very nature, so it's really up to you which you prefer. Also, depending on the aerobics classes on offer at your local gym, you may find a different demographic in the aqua classes - there are often more pregnant women, over 50s and overweight participants than in a regular aerobics, particularly high impact aerobics.
In any class, you'll find a huge mix of abilities, fitness levels and shapes and sizes - just check in advance what level the class is aimed at. Let the instructor know that you are new to group exercise and he or she will advise you (probably tell you to take the low options to begin with and drink plenty of water - and more than likely follow up with you at the end of the class as well).
If you need to take easier options, remember that in a group fitness situation nobody is going to give you a hard time about doing so, and in fact it's unlikely anyone will notice - they're either watching the instructor or watching themselves in the mirror. There just isn't time to watch the other participants in the class.
In terms of my personal recommendation, I have to admit I'm not a big aqua fan for some of the same reasons as you (too slow/can't keep the beat underwater/can't hear the instructor over all the flipping splashing/instructor never looks like they're doing any work at all...because if they're poolside they're usually not) and I'd go for 'land' aerobics every time. Why not try one or two classes with different instructors and see which you like best? Often the instructor can make or break the class. I have people who will only come to my classes and nobody else's. Equally, I'm sure there are people who won't go to mine because they prefer another instructor's style.
Kx