Ya, I have used phpbb3 SMF comes with poor design I know there many templates but they doesn't compatible with all mods such as pretty URL, ad mod Its default theme is boring Where at another hands, there are thousands of good template of PHPbb3
You are right, thats the only reason i had to go with phpbb3 for my computer forum, I didn't find a better theme in smf than the one i got for phpbb3. But installing mods in phpbb3 is a time consuming process. And for the conversion, i got this while searching, see this http://www.phpbb.com/community/viewtopic.php?f=65&t=589439 it may help u
With that said phpBB doesn't even have a mod installer by default. So you will be inserting any modification codes anyway if not more. How so? Have you seen the new SMF2.0 curve announcement? There are over a thousand themes already available on the SMF theme site. If you wish to go with phpBB3 than go for it. But just sayin' its going to be harder for you.
shadow82x pretty much said it, all themes will work with mods, they just need to be edited. do yourself a favor and learn a little, and stay with what you got.
phpBB will tell you how to move TO phpBB But i recommend you DONT move away from SMF, its more easier and flexible...