I, too, have seen a wide range of suits on any single dive due to peoples' varying tolerances to cold. We own several suits and take the one(s) that will suit the diving we are doing on any given trip.
Rent-to-try sounds like a good idea--especially to see what you like in terms of thickness. However, you may not get experience top of the line materials. For example, Henderson Gold suits, due to their cost, may not be for rent. However, if you like 3mm for general diving, you can get a Henderson Gold in 3mm after you ascertain that. We like the Gold because the inner material makes getting into the suit easier--especially when it is wet. There are other materials, like "Titanium" that we have heard are similar, but we have not tried them.
Custom is expensive, but for some is the best way to go. For example, Barbara had to have longer sleeves and legs than she could buy off the rack. I can usually fit off the rack things, but sometimes the arms on wet suits are too short for me. One of the nice options on a custom suit is zippers for the arms and legs--really lets you get into the suit easily and makes for a snug fit. Most mfgs will make a custom suit--ordered thru your dive shop--BUT THE COST IS USUALLY HIGH. That said, if you pick the right options, you will use it with joy for a lot of years.
FIT--remember that you want it snug so there is minimum water movement though the suit. This can sometimes be very uncomfortable when trying on suits for the first time. On the other hand, do not strangle yourself!
If you planning a lot of Carib diving, want more than a skin (which really only provides protection against jelly fish, etc. and not much warmth), try a fleece like O'Niell's. They are easy to get into--even on a humid day--provide some warmth, and are very comfortable.
Joewr
