I think the whole concept of it being a big plunge to switch styles is overhyped. I can't see that moving from Bronze Smooth to Bronze Standard is any more difficult than moving from Bronze Smooth to Silver Smooth, yet no one calls moving up a level in Smooth "a big plunge." I'm saying this not to make fun of what you said, but rather to reassure you that in the end it's not that big a deal after all!
Personally, I'd start with the Standard Foxtrot. It's the least like any of the Bronze Smooth you've done so far, but it's the foundation of all the "swinging" Standard dances. Also, Bronze Standard Foxtrot is a lot like Silver Smooth Foxtrot in turns of basic movements and passing of the feet.
If you want to stick with the familliar, though, then start with Waltz. I wouldn't bother with Standard Viennese Waltz because it's a subset of what you've probably already learned in Smooth.
Another reason for starting with Standard Foxtrot is that it's addictive. It's a difficult dance (but not any more difficult than starting Silver Smooth Foxtrot) but the payoff is enormous in terms of how great you'll feel as you discover new things about dancing and improve your dancing and learn new techniques and patterns.
