The theory is that the insulation will prevent the tank surface from getting cold. Well, insulation slows the heat transfer, but just that, slows it down, not stops it. Depending on how often the toilet is flushed, you might keep replenishing it with cold water, and you'll cool off the tank regardless. It will only work if it is flushed infrequently. Then, less of the tank gets cold soaked, and the room heat can warm up the water enough to prevent it from causing condensation at least for very long.
Before you spend money on any new toilet, check the performance testing on Terry's home page. No sense getting a new toilet only to find out that you need to flush it 2-3 times to evacuate the crud - you'll just make your problem worse, even with insulation.