most companies offer model numbers that are similar but represent their actual size in cm. (js310, air deck 310, etc)

find the size boat you need first. compare tube size (the bigger the tube, the dryer the ride). pick a material (hypalon is stronger than pvc). pick a floor type (wood, air or alluminum). get a good comparison on their weight capacity.
good site to compare side by side models is
www.campisanomarine.com

good luck!