Many of my customers in the south purchase from Walex Chemical 800-338-3155 they offer several fragrances. One big thing to do in the high heat is to add a few more gallons of water with each service to fit the evaporation and keep the chemical active covering all waste build up in the tank. A little extra water will help keep your unit smelling better during those hot summers.
Hope this helps.
There is not really any certain fragrance that works better. I would tell you to do the exact same thing that Steve said in the other reply. The longer you can keep the waste below the water line the better.