A good quality 6 bulb will put out more light than a 8 bulb so it really depends on the brand and reflectors make a huge difference. Adding a good single bulb reflector to a t5 can increase the light out put by 300% over no reflector. That being said, I looked that light up and don't think the reflectors are that great, very narrow. There's at least 3 better lights for sale here used in this forum that are 48" if your willing to go that route. What size is your tank and what are you trying to grow?
I grew sps with pretty good color with a four bulb ebay fixture (individual reflectors though) over a 75 gal. I also had ati bulbs in it which I think made a huge difference. What size tank will it go over? I agree with the above post I'd rather a high quality 6bulb fixture.
I use an 8 bulb tekkie with all ATI bulbs... bulb quality definitely helps... i cant comment on the quality of the OPs product link, but I dont think they have the reputation taht the teklight has... and of course, the ATI fixtures are awesome, but twice the price of even the tekkies... but as for the original question of 8 bulbs versus 6 bulbs? i will say 8 everytime... but thats jus me... again, i use and 8 bulb, and kinda wish it was 10... overkill or not... it prevents me from having to add lighting when i upgrade to a wider tank or a tank that is up to a foot longer... anyways, i say 8... but the quality of the fizture has ALOT to do with it... GL with whatever you choose!
I have a 72" 165 gal tank. I have some corals. Just like to have the light in case I ended up needing more light. Not going to do hard corals though. Right now my lighting is a 24" coralife power compact and a 65k 48" t8 shoplight. Thanks