At the c.i., rpm, and hp level you're targeting, I'd say either will give you more than you need, so I'd go with the less expensive of the two, and talk to CStraub about a cam. He may be able to help you with a set of Canfields, as well, though I'm not sure, and don't want to speak for him on that. You can see how well they worked on my build, so I don't think there will be any problem with them on yours. Even at 310cc's, they might be on the big side. The AFR's, at 305, aren't going to be any more friendly. 5cc in port volume isn't going to make any difference, here. The Canfields are real strong on the ex. side, which, IMO, is a good thing.
Just looked at AFR's website...you'll never get the flow numbers they're advertising. They flow on a 4.600 bore. Your 4.31 bore is going to shroud the valves quite a bit. Canfield flows on a 4.500 bore, so keep that in mind, as well. You can look at the "canfield 310's" thread here in gearheads, where I posted my numbers.