To a point IMO single railing is better, higher wattage such as 1000w+, i'd still give it to split railing
alot of the newer companies such as pc power&cooling are single rail companies. you want a gaming psu, get a nice single rail. Usually if your going to get a split rail, you get something with more wattage so your sure all your railings are at high amps.
I had an 800w kingwin running 4 rails, 22a 20a 22a 25a i think, or maybe 20a 20a 20a 25a.
not every rail will be at full potential. They will have there own limited peaks for each main component in your system.
But, i ran that psu on dual 3870's, yea you might think of it as an overkill but, it wasn't. had a high wattage but, that was just for how long it would last. i've had it for 3 years now, running a gtx 285 on it. No problems what so ever. Thats because you have to use 2 rails into a single gfx card. that leaves 2 rails left for the rest of the computer.
All in all, single railage is the newest thing but, keeping things limited so you dont get an over amperage is much safer, even though they say theres some kind of special thing to keep your psu from overloading single railage, it's still possible for it to happen.
This isnt putting on another war against multi rails vs. single rails, there both good psu's to a point.
Single rails just seems to be more new and is getting better.
For multi-rail psu's you can't just add the 12V+ rails to get the total amps. Usually the psu manufacturer will specify the total 12V+ watts available for all rails. To find the amps, divide the watts by 12. But for single-rail psu's, the amps for the rail is the total 12V+ amps available.