We really need a Flash FAQ sticky, I keep meaning to write one but I never seem to find the time. There is a ton of good info out there on this topic but it is all over the place
All the 4/3 & m43 first party flashes have the same spec, and they are all capable of TTL attached to the shoe (with one exception the Kodak flash which does not adhere to the m43 spec).
Other than the obvious differences around guide # and recycle speed the primary difference is whether they can use the wireless RC functions. Any of the Olympus 4/3 flashes with the "R" can be controlled remotely. The non-R flashes only work in the hot shoe or via a cable. The 600R is the only flash other than the little built in flashes that can act as a remote commander.
If you only want to use the flash in the hot shoe or via a cable and you want lots of light, the FL50 is a great value used.
Personally I have a FL600r and two fl36rs. My oldest son has an E-PL2 and an FL36, he only uses it on the camera and it works great.
I am not really familiar with the Panasonic flashes although most of them appear to be virtually identical to the Olympus ones. The new DMW-580L looks to be very interesting, it seems to be an updated FL50r.
One thing to be aware of about the FL600r, the LED light acts as the active slave notification and it is very bright, really annoyingly bright. The FL36r and FL50r both use red LEDs which seem to be more appropriate.
HTH
