I like reading this guy:
He's just a guy out there who likes to shoot, and make pratical tests with his gear....and it's not always praises either like Pop. Photography.. Seriously, after they gave Tamron's 200-400mm f5.6 push pull lens, the "Lens of the Year" award,...I gave up on them completely!
Also go see fredmiranda.com,...but take everything with a grain of salt.. because you are reading all sorts of opinions, from people with all levels of experience.....which is why I like the consistancy of the guy above.. BTW, if you see that page, and scroll down, you'll find tons of other info too..
Another independent is www.photodo.com
They've tested a LOT of lenses, but haven't tested anything in years....not sure why..
Regarding the f4 vs f2.8,... I guess it all depends on what you are shooting.. The 1mm difference is more noticable in the wider end than 5mm in the longer end between the two..
I shoot with 2.8 glass, because I need to in my line of work.. In fact, I almost never ever use f4.0 in my work.. I usually skip it all together..and go to f5.6 if I need to stop down a bit,...or I'll be shooting wide open.. Also,..the 2.8 lens is a brighter lens to see through, and it takes advantage of the cross hairs AF sensors where f4 lenses don't.. The 2,8 costs more, yes,..and it costs more to insure,...and to repair..so keep those things in mind too..
Quality between the two,.. I couldn't tell you,... I'm still using my 17-35mm f2.8 L lens that I bought in 96.. It still works pretty well, even on the 5D..
Shot this with my 17-35mm f2.8 L lens on my 10D, RAW, and corrected for mild CA. It was used in a 4 column magazine ad: