i have 550VX, i was deciding between the two as well.
depends on who you ask if 3 12V rails are better, or a single heavy duty 12V rail is better. i dont have a lot of peripherals, and therefore i think one single rail with loads of amps is better than multiple rails with small amperage.
i preffered the heavy single rail.
41A (550vx) vs 18A x 3 (520hx note: this doesn't mean you have 52A on the 12V).
either one will be fine. but, i would go with 550vx or another good name brand with heavy single rail if you are going to SLI cards. thats why i bought one.
plus, modular psu's are good, buy there is always the chance that the pins wil corrode.
they are both good PSU's.