580 ex or 580 exII?

I have never used a flash before, but am looking to get one to fill out my gear. Which flash is best with the XTI?

well the mark II is newer, and has a metal foot, and is weather proof, and has a metering 'eye' (?).....

I would go with the II

Mk2 has silent recycling (which can be a plus or minus), metal foot & new mounting mechanism (again, plus or minus; some report problems with new foot; if you hit the flash, would you rather the camera break or the flash foot break?), annoying menu item for slave/master (definite minus), faster recycling (plus), better battery door (plus), PC sync port (definite plus) and non-TTL autoflash (definite plus). It's also much more expensive used, I got my 580EX Mk1 for $250.

I would recommend 580ex I and not II.

Though you can always overbuy and be happy with a 10% of the features, even on my 1D Series cameras (Mk II and III ) the 580ex works very well. Would I eventually want a II so that I can make use of TTL II, and a few other really nice features? Sure, but I am not in a hurry.

But both flashes have the same relative power (output of light). Features are the only difference, the Mk II released almost at the same time with the 1D Mk III and TTL II if I am not mistaken.

So buck for buck, and performance based, you can save a lot of money putting a first generation 580ex on your XTi than you would a 580ex II.

PhotoJourno wrote in post #5780102
the Mk II released almost at the same time with the 1D Mk III and TTL II if I am not mistaken.

You're correct that the 580EX II was released with the 1D Mk III. When used together, you can control certain flash functions via the camera menu (I still haven't figured out what's so great about this). The 40D and the Rebel XSi have similar compatibility.

E-TTL II flash metering was was first introduced with the 1D Mk II in 2004. And every EX series Speedlite, including those produced before then, are 100% compatible with E-TTL II.