Are you having trouble with the front too? If not, how concerned about the appearance of them looking like a matched pair are you?
Myself, if the front wheel is okay and not giving issues, then I'd put all my money into a new rear wheel with a much higher spoke count.
If the shop is ordering the wheel, go by what their person says will work for you. They probably can tell a little more about your riding habits and stuff than we can. But what ever you get, have it looked at around the 300 mile mark or 3 months which every comes first. Some good wheels will need attention then. After that, you can just check the spokes yourself to make sure none seems excessively looser than the others. No gauge necessary.