It is a bit like planetary, a bigger aperture can reveal better detail, but seeing can often mean you see more in the smaller scope.
I have a 60mm PST and have done an 80mm mod also and get views as good as any Lunt 60 I have seen, but for photography the sweetspot is wider in the Lunt, so it may be a better choice.
The 90mm scopes are *serious* money.
Yes the focal length goes up with aperture so you get less of the Sun on the CCD.
You may want to go over to Solarchat and have a chat, lots of Solar scope owners over there.
http://solarchat.natca.net/index.php/en/