I have a 1 bedroom home (half of a duplex) that's roughly 650 s.f. with a 1.5 ton straight cooling system, with electric heat, in Central Florida (so the cooling is far more used than the heating!). The air handler is 20+ years old and the condenser is about 12 years old so I'm getting to the point where I need to plan for a new system.

First, I assume that in general a heat pump is a better choice than a straight cooling system?

Second and more important, I had a sales guy tell me that I should consider a two-stage heat pump to help with reducing humidity. Since I'm in Central Florida, we have pretty high humidity year round. It's not such a big deal inside during the long summertime because the AC system pulls out enough moisture to keep the humidity down. But during the cooler months when I'm not running the AC, humidity gets to be more of an issue and the electric heating (when used periodically) doesn't do anything to help that.

The sales guy told me that with a two-stage heat pump, it could run on the lower speed to help reduce humidity; during the hot summer it would shift up frequently to the higher speed to cool more. In cooler months it would stay on the low speed and that would help to pull the moisture out of the air.

Am I understanding this correctly, and more specifically - is a two-stage heat pump system a good choice for my situation? Thanks!

You are understanding it pretty well. The two stage equipment will give you most of your cooling at a lower operating cost. It also runs longer at the first stage which in turn will help with the humidity issue. Too large of an air conditioner tends to cool very fast then shut off. That fast cycling doesn't adequately remove the humidity.
Some equipment has a setting to keep the fan on 24/7 at a reduced speed until the cooling is called for. This regulates both temperature and humidity better.