I dive Northern Florida a lot, I also dive in Colorado. I was trying to cover the gambit of temperatures you are probably looking at too. I ended up with a 5mm two piece. If I dive Fl in the cold months (Nov - Feb) the farmer john & shorty are good to about 55 degrees for me. In March when it warmes up to the 60s (65+ @ 60fsw) I can get away with the shorty sometimes. Although I will dive in both in some places. I haven't dove after March, so I don't know how warm the water actually gets down there. In Colorado, the shorty and farmer johns together work just fine. Water temps are in the 50s to low 60s, but I move around enough that I don't get too cold.
The hat and gloves are the key for me. I dive mostly in a 3mm beany with 2.5mm gloves. My hands get cold in the 50s water, so I'll change to a 5mm set of gloves then. I'm hot blooded too, but have never got hot in my suit (under water). I dove in a lot of rentals before I got it. Most of which were XLs, I found out they were actually too big and bought a Large. The fit on land for me was different than when in water. Compression is a big deal. I also found the suit is not as tight now that I have dove in it a while. Keep that in mind when trying on suits.
I don't know what Mike S is talking about, but it sounds like something to check into. Don't come at it from an I'm tough, I can take it. Nothing spoils a dive faster than getting cold. I tend to wear shorts most of the winter here in CO, but getting cold under water is one thing I don't want to do. Cooling off is easy, getting warm is not.
Greg