QD uses the same transfer case as the QT II but adds a limited slip rear diff and a bit of software (your QD may have had ELSD on both axles). So the advantage/disadvantage is what you think a rear limited slip diff gets you. Limied slip uses up less power (unlike brake traction control) so it gets more power to the road under wheel spin or imminent wheel spin conditions. Some here have noticed just a bit more wheel spin without QD. QD would be most usefull in deep snow, mud, or rock crawling.... or autocrossing
The summit V8 comes std with 20s and QD. You can get the ORA with its QD and 18s and then 20s separately. 18s are a good choice if you don't want to bang up the 20s in the winter or off road. The OEM GY 20s are OK but not that great anyway, you may want something different. Many here own two sets of wheels/tires.