I have no experience with that brand.
Larger SSDs tend to run faster in the same series, so I'd go with the bigger one.
Video: We have had excellent success with the xw6400 driving a Quadro FX1800 in the top PCIe video slot, and a FX3700 fed PCIe supplemental power by one of the power supply 6-pin plugs. The xw6400 only has Gen I PCIe technology, and its lower video slot does not feed up normal 75W to the card in it. Thus, we set the FX1800 in the upper slot (which feeds enough power up through the slot) and supplement the card that has the 6-pin receptacle via that cable and put it in the lower wattage video card slot below. Those two cards run on the same drivers. We've been running these workstations with 4 monitors for years now this way.
The xw6600 has its two video slots both as PCIe gen II, and both slots feed up a full 75W to the card.... for those I like the Quadro 2000 in the upper slot and the Quadro 2000D in the lower slot for our needs. Two of the Quadro 2000 are fine too, and less expensive. In the faster Z600 gen II I like two of the K2000 cards, and same for the Z620. We are not playing games on these, or designing running jet engines on the fly.....