Well it would perform slower, but not noticeably slower. I would go with the 2x4 just because the system would benefit from the faster RAM speeds. It seems that the build probably won't be upgraded, so it's better to just get 2x4.
Other than that, if you can find a better power supply for the same price, get the better power supply. Thermaltake is just one of those iffy companies. I'm sure it would be fine but it doesn't hurt to look.