Have a Midwest on my 16 and 17 with no complaints. Sturdy, easy install and plenty of added real estate. Haven't used the other manufacturers but went with MWI based on plenty of feedback on this forum.
Imho, they are all good. It just depends on what you prefer. The pws srx top rail is lower than top rail of the scar, this helps if you have a laser like Travis Hailey's setup (just look up his scar video on YouTube). The Midwest matches the top rail and they have a version that you can replace the front site with more rail space. Then there is the branham 2" rail that just extends to just the end of the front site iirc. Its the lightest but also gives less rail space.
Also this thread has some more info on extended rails http://fnforum.net/forums/fn-scar-17s/48227-lightest-extended-rail-scar-17-a.html
I think most of the rails are good but they each have their pluses and minuses....based on shooter preference. I have tried both the PWS and the FN rails. I have also modified, cut up, ground down and otherwise massaged each rail trying to get the perfect balance of performance vs weight. I just recently ended up going back to a stock PWS rail section. For me, the PWS has a better feel for my forward overhand grip. I tried shortening it to save weight but the weight savings was not worth the loss of length that shrouded the barrel when shooting off supports.
Just my two cents worth
I had and liked the full Midwest Industries rail on my 17 and loved it. I got carried away with adding stuff to it and didn't really think about why I bought this gun. I bought it because it was a light and reliable .308 semi. I took it off and went with the shorter ones with the qd. I want to keep it as short as possible.
I installed the first version of the MI SCAR Rail. I wanted an extended rail primarily for an extended barrel shroud, not because I needed a lot of rail space. I thought about the Vltor, but by the time I added the extension to get the same amount of barrel protection, it would have been a lot more money. The first version of the MI rail does not require removal of the factory front sight, and addition of another expensive folder BUIS. The MI rail installed relatively easily, just by reading and following the directions included. It adds about 8 oz to the weapon. But, when shooting off a bag or around barricades, my barrel does not touch anymore and through off the POI.