Amos - Geoff the Duck and others above have pretty well nailed it;   It just doesn't work too well to try and use one banjo for both styles.

I looked at the pop-off resonator solution for a bit, but came to the conclusion that to get the frailing sound you also need medium to heavy strings, a sock stuffed in the back, usually a different type of head - like a skin head, and sometimes a certain type of bridge and tailpiece (mileage may vary).   

Those acoutrements do not get you the bluegrass sound - where you usually want light strings, the resonator, a brighter sounding mylar head, and carefully selected bridge and tailpiece.

I ultimately found an old Vega for frailing, and then, for bluegrass, acquired one of the new breed of Gold Stars (NOT Gold Tone) which Janet Davis Music (on web) had as a special at $995. It required changing out the tuning pegs, which were really crappy, but it has a snap that impressed a friend of mine who plays both a Stelling and a Wildwood. I'm happy with both.