Howdy,
I too have been searching for just such a site for several years, and have come up short. I wish the fish magazines would do more comparative studies, but they just don't. I think they avoid such studies because it might alienate advertisers, which they count on to keep themselves afloat.
Your best bet to find comparative information is on boards such as this. Conduct a search on past posts regarding the type of equipment you want to find out more about. That is what I did when I was looking to upgrade my old seaclone skimmer to a new "advanced" unit. I also posted several comparative questions on a few boards, and was pleased with both the quantity and quality of the responses.
You will find that alot of the hobbyist on this board have extensive knowledge with several different brands in each category. I have used both AquaC and Euroreef skimmers for several years and can tell you the pros and cons of each one. I can also compare serveral types of lighting setups, as I have used everything from Home Depot NO ballasts to different commercial MH setups, and quite a few power compact setups. I can't give you categorical data on each item, but I can give you my personal inferences and observations.
Good luck,
Scott.