Welcome to shuttertalk, and welcome to the DRXT/350D club
I never really got a chance to play much with a 420EX, but I bought a 430EX the other week and love it.
On paper there are *loads* of functionality differences, the manual control being just one of them (and a very useful one). But other really handy things are being able to adjust the flash exposure compensation, 2nd curtain sync and high-speed sync without having to dig through the 350D's menus (it is one-button press on the flash). It also gives you handy information when you lock focus, such as an estimate of the distance the flash will be effective over (ie 18m).
And the E-TTL II functionality simply means it is smarter than the 420EX, can make use of ALL the information the camera and lens can provide to it, and should simply do a better job of metering the flash.
For the small difference in price between the two, I would DEFINATELY suggest getting the 430EX over the 420EX.
If you're a newbie as you say... I hope you're planning on getting a lot of practice in before looking for gigs as a wedding photographer. They aren't the sort of situations where you want to be learning on the job as there are no second chances. But certainly you can do plenty of learning at weddings if you aren't the official photographer, and they are great opportunities for lovely candids too.
Hope your enjoying your XT.. and you'll have to post some pics for us to look at