Two quick queries

Whether it is worth it is really up to debate and will come down to your preferences.  The Aventum II has space for two PSUs, but if you're sticking to GTX 980's, they are unneccesarry.  If you're going to keep the GPUs operating at factory speeds, then you don't need more than a 1200W PSU.  If you're going to start to overclock them, then you'll want 1500-1600W, but you can get that in both the Aventum II and III.

As for your question about the GPUs, it again comes back to overclocking.  Since you most likely are not going to do liquid nitrogen cooling (defeating the purpose of getting a computer like this), then cards like the EVGA Classified's and ASUS Matrix Platinum's aren't really worth it as you cannot use the extra power capacity without extreme cooling.  Also, the ASUS STRIX card, for example, does not have any overclock on it and may not have as good a chip as reference (ASUS, as other manufacturers do, bin their processors, screening them for the better ones, placing the best ones on the Matrix Platinum, next best on the STRIX OC, and worst ones on the STRIX; reference designs are manufacturered at an ODM elsewhere and delivered complete to whichever company (ASUS, EVGA, Gigabyte, you name it) and then packaged with the branding, so those chips are luck of the draw).  The STRIX may still have better power circuitry than reference, as well as the ASUS developed cooling solution.  The ASUS STRIX OC and Gigabyte G1 Windforce have been binned and both have mild overclocks on them.  The EVGA FTW has base clock speeds equal to that of the boost clock speeds on both of those cards.  The fastest out of the box GPU is the GALAX Hall of Fame card (also binned).