www.robgalbraith.com says there's about <= 0.5 stops difference between the 430EX II and 580EX II
That can be a big one if you're shooting all day or with a LOT of full power pops. Most times it won't be a concern though.
While on a FlexTT5 I don't think the weather sealing will apply
I currently use MiniTT1s with the AC3 controler with FlexTT5s and 430EX IIs. I used to use 580EX IIs but prefer the 430EX IIs because they play much more nicely with the PocketWizard system.
I like having ETTL in my back pocket but I only use it in a run and gun type situation which makes up about 2% of the time. I never use it for portraiture. I find manual to be much more controlable and predictable. Joe McNally does a pretty fine job with TTL though :-)
Read this blog post by "Strobist" Q&A: Controlling the Sun When Using Flash - A Comparative Guide. It may change how you think about shallow DoF in bright sunlight.
For me, the ability to change power settings from the camera via the AC3 controller is worth the total ($$$$$) expense of the PocketWizard Flex system. If setting the power at the flash is not as big of a deal then I would highly recommend CyberSyncs from Paul C. Buff. They are moderately priced and highly reliable.