First thing I would say is don't buy anything until you have the Unicorn in your hands. Things can still change of course, so we'll want to make sure 100% that things are compatible, and the only way to be 100% is to have the bike in hand. I already made the mistake of pre-buying SRAM 52/36T chainrings for this bike, which are not compatible with the FSA crankset.
To answer some of your questions, yes there are different versions of the DZero. There is the Quarq-branded version (carbon or aluminum cranks arms), the SRAM Red crank version (carbon crank arms), or just the DZero spider (no crank arms). In our case, since the FSA crankset has a completely different chainring bolt standard, we will need to purchase the entire crankset (spider, crank arms, chainrings), and for me the obvious choice is the SRAM Red Quarq DZero (Your choice on crank arm length, 110BCD bolt pattern, BB30 spindle).
As for whether the Quarq is "better" than the FSA/Power2Max, this is completely subjective. I don't think I've ever stated that the Quarq is better than the FSA, only that it is a better aesthetic and component match for the Unicorn Pro/X bike. I also have owned 2 Quarq power meters over the past 10 years and can say their products and support are outstanding.
The stated accuracy of these PMs is 2% for the FSA, and 1.5% for the Quarq DZero, so I'm sure the FSA/Power2Max will be plenty accurate enough in terms of measuring power output. Unless you feel strongly about the aesthetic and component match as I do, I would suggest keeping the FSA crankset, it's just not going to be worth the hassle to upgrade just for the sake of having "better" components.