RAMPS 1.4, 1.5 or 1.6? I'm about to build a Prusa i3 dolly. I am confused whether to use RAMPS 1.4 or 1.5 or 1.6.

What is the big difference? Is it only the MOSFETs and the poly-fuses? If that is the case, would it be advisable to upgrade a RAMPS 1.4 board (replacing the MOSFETs, connectors, and fuses)?
The biggest issue with RAMPS 1.4 (and 1.5) is the power connector is prone to melting/burning, this appears to be fixed on 1.6 with the use of screw terminal blocks. I've used RAMPS 1.4 with both 12v and 24v power supplies and never have had any issues with the fuses or the power connector but mine have only come from Ultimachine or RepRapDiscount.  A RAMPS 1.4 with power connectors and fuses replaced with those from a reputable dealer (Digikey, Newark, Allied) will likely be fine, you can even remove the power connector and solder the wires directly to the board if you don't need the ability to unplug them.

Whichever you go with, make sure the screws are tight and never tin the wires going into the power connectors.