I think there are many types of PSU on the market because there's so many configurations of high-end computers as well... Since you're talking about a modular PSU, I assume mid to high-end computer since you normally can't get ~250W modular PSUs.
ilya explained the multiple vs single rail quite well, so I'll chime in on the modular.
For modular vs. non-modular, you can save money with non-modular (e.g. Seasonic S series vs. M series) and you don't have to worry about a loose connection on the PSU end. Note getting such a loose connection is rare but it does happen. If you have a cramped case or bad cable management, then you'll want modular unless the added length from the modules will give you clearance issues.
In any case, before buying a power supply, look at a good review on it that FULLY loads the PSU with a load tester and measures with an oscilloscope. You can have a pretty-looking single-rail fully modular PSU but it's no good if it smokes and damages half the components it was connected to in the process.
__________________  "The computer programmer says they should drive the car around the block and see if the tire fixes itself." [src] |