1602 is a wafer, 1588 is electronic thermostat. I'm not sure if the 1588 also always comes with a fan or if that's optional but a basic 1602 doesn't. However both turner and fan can be added to a 1602 so it's not really a big difference. The different thermostat though does make the 1588 better. It will heat more accurately, require less fiddling with the controls, and have smaller temp swings. If you have the money it's worth it. If you don't I've been hatching over 90% consistently on my own eggs with my 1602 and no fan or turner so I see no reason to spend more money on a better model.
I have one of each. I like the 1588 better for a hatcher since the big window lets me see the babies and the rocking eggs. A lot of people had to change the temp setting on their 1588 to get it up to 100 (and stay there), including me. If you're only going to have one, go for one with the picture window -- you'll appreciate it come hatch day. In addition to the 1588, the 1583 also has the picture window but uses the wafer thermostat rather than being preset. Price-wise, it's between the other two.
