SMF can be skinned, yes, but the mod community is not very large, in addition there are not as many skins available with SMF as there are phpBB.

As far as conversion, if I remember correctly, you will have one pretty crucial hurdle to jump moving from SMF to vB that you would not have moving from phpBB to vB and that is the passwords. Passwords are stored differently with SMF than vB...I'm pretty sure there is no way to maintain the passwords, so you would have to make all members reset their passwords after the conversion. I've moved from IPB to vB before and had this same problem...it was definitely a setback, but it's not going to kill your forums.

If that concerns you...and if the skins concern you, then you should go with phpBB. More skins and no need to worry about that password issue.