Personally, I'd go with a 270EXII (which can also be a slave). The 320 will certainly have a bit more power than the other options, but I don't think it's that much smaller than the 580/550.
I just picked up the original 270 ex for just a touch of fill flash with my 5D II. I have been very happy with the results. It is unobtrusive and I actually like how the light fall-off works. I have vivitars and pocket wizards for the off camera work, so I saved a few bucks and bought the non-slave 270. Best Buy had it for $95.00.
I have both a pair of 580EXs and a 320ex. I bought it to use as a slave and to carry when I need something for fill flash and don't want to lug around the larger flashes. I am a "bouncer" and like the tilt and swivel capabilities of the 320ex.
I have a Canon sx1 IS that I use when biking and the 320ex fits nicely into a small bag with it if I should need some flash along the way.
It seems considerably smaller than the 580EX especially in hand. I like the small size of the flash but its reduced power compared to a 580EX is noticeable especially when bouncing the flash. I find that the LED light is very limited in its ability to light a scene having tried it with both a 7D and 5D II for video. Its controls are very straight forwarard. Overall I would call it an good flash but not a great one.
looking at the comparison makes the 320ex look quite large compared to the 580 . but on the one occasion I saw one in a store window (out of the corner of my eye) i thought it looked quite small. but then I didnt have a 580 to compare the size with.
The LED thing is of pretty much nop interest to me . same goes for the remote release . but the wireless function is.
I have an offer on 1 thats pretty dam good. quite a bit less than I see 270ex2 heads go for . more what I see the non wireless 270 go for(maybe slightl more)
I'm only going to use it for a bit of fill in the main as if i want to use flash more I'll have the 580 with me (more likely all of them)
I bought a 320EX for use with my 5D II and 40D
I am getting good exposures
and like the ability to turn the head for various bounce options
the 320EX works well for fill flash and 2nd shutter curtain sync
The smaller size of the 320EX works for me. Much easier to bring along than the 580EX II.
I've got a 320 and use it 75% of the time (over my 580) due to its convenient size and weight. It's also great when using my 60d because I can instantly set up off camera. I would definitely recommend it over the 270.
Here's a better size comparison (ignore the reversed head)
I have one. I rarely use flash and I only got it because it was relatively cheap. The thing just works. And when you want to bounce/fill you can turn the ISO up.
