The WF-188 has two charge rates, in order to cater for cells of different capacities, and two different termination voltages to cater for LiCo/LiMn cells and LiFe cells. These parameters are altered with two, two position slide switches.
You have two bay cradle for all cells sizes, and don't have the fake (not connected) side bays for 16340.
The cradle is open, and the charger wider than normal, allowing for the charging of larger cells than the WF-139 would accept - 25500, 26500, 26650, 32600, 32650 etc.
Those are pretty much the differences and benefits. Neither is a particularly good charger. Unless you really need a dual bay charger for 32600/32650, I'd be tempted to look elsewhere, though a better charger will of course cost more. There's the ever popular Pila charger, or the 4Seven single bay charger if you are ever going to need to charge 25500, 26500 or 26650 cells. That has gained itself a good reputation in a short time. XTAR charges are also gaining themselves a good reputation.