The pro T is a bigger version of the press T... Press for press photogs who need to be more stealthy and nimble... pro T for wedding guys or photogs that need larger cameras to compensate for whatever. Some think the pro T gives better seperation and it does but who cares? We usually bouce this anyway right? And if you are shooting straight on its really fine... go with the press T, it's a great bracket for a 20d/grip... just get the right anti twist plate.. you will need it. Its a much bigger headache trying to get TTL work than the bracket... try using auto thyrister strobe instead, its more accurate.
The whole point of a bracket is to get the flash directly above the lens in both the horizontal and vertical shooting position. With a 10D/20D/5D and their respective vertical grips the Press-T WILL NOT hold the flash over the len in the vertical shooting position, its arm is too short. Subsequently a Press-T is hardly a Great Bracket for any of these camera/grips.
From my experience bouncing isn't always possible (ten metre ceilings?) so my bracket must give me the shadowless verticals even when the flash is pointed directly at the subject. Without this capability I'm just kidding myself that the bracket has some function, and so is a press shooter.
With the 20D and E-TTl 2 the flash automation is OK. It might require a tweak of exposure compensation but it's miles beyond E-TTL. I think that an automatic flash will work well and something like a Vivitar 283/285 is a good choice. But a 550/580 is the first step on the road to Canon wireless and that's something that you can't do with an Automatic flash. Effortless fill flash is also a benefit of a TTL flash. I set the ambient light with the camera, turn on the flash and pretty much forget about it. As I (or the camera) change apertures it will compensate. With an Automatic flash if I've selected f8 on the flash, then the aperture on the camera had better be f8!
By the way when you take a flash picture the capacitor that has stored the power dumps it to the flash tube. On full manual all of that power is sent to the flash tube. On automatic settings the flash may require only a small part of the capacitors stored power and rather than dumping it the auto thyrister circuit returns it to the capacitor. This saves battery power and improves recycle times. I had a Strobonar Automatic flash without the thyrister and when I used f2.8 it would dump the extra power with a noise that sounded like a snapping whip! In any case Automatic flashes like a Vivitar 283 has that auto thyrister circuitry but so does just about EVERY camera mounted flash made these days. Auto Thyrister was the New Great Thing in about 1976 or so and there's a good chance that an old flash has Auto Thyrister on the front of the flash the same way that a new car might have 4WD under the model name. But it's not something that differentiates an Automatic flash from a TTL type.
"There's never time to do it right. But there's always time to do it over."
Canon 5D, 50D; 16-35 f2.8L, 24-105 f4L IS, 50 f1.4, 100 f2.8 Macro, 70-200 f2.8L, 300mm f2.8L IS.