i think you would be fine either way, but 8 is better than 6, especially for sps. im not too familiar with either fixture besides knowing they're nice, but im sure the 8bulb fixture would give a wider spread.

on the other hand the 6 bulb might be better for your lps which will probably be at the bottom front of the tank... the light would taper off and not be so direct on them, where as it would be just as intense on the sps which are probably towards the top-middle and top-back. its all about what you want to do, but again i think you could make either one work.