I went the other direction: I was an obsessive, compulsive 5-string banjo player for close to 30 years before becoming an obsessive compulsive fingerstyle/classical guitar player (which started about 8 years ago).
I'm a banjo snob, and I tend to view 5-string banjos as the real deal, and everything else as being inferior. OK, yes, tenor banjos are legitimate instruments (but really, whole different beasts), as are 6 string banjos (I guess), but a 5-string banjo is something special that presents some unique opportunities that other (inferior
) forms of banjo lack. A 6-string banjo could probably be a a lot of fun, but if you want a really genuine, traditional banjo sound I don't think you are going to get this without a 5-string banjo. If you want to sound like Pete Seeger or Earl Scruggs or Tony Trischka or Bela Fleck, get a 5 string banjo. If you want to play Foggy Mountain Breakdown (like in the Bonnie and Clyde movie) or The Ballad of Jed Clampett (theme song from The Beverly Hillbillies - people love it when I play this), get a 5-string banjo. If you want to sound like the old traditional clawhammer/frailing style players, get a 5-string banjo. Yes, I'm a banjo snob.
A 5-string banjo has very small intervals between the strings (in standard G tuning anyway), and past the 5th fret (which is where the 5th string starts) the 1st and 5th strings are the same note (and past a certain point on the neck most notes can be played in 4 or 5 different places on the neck - I'm talking about the exact same pitch here). This allows for really really smooth runs of notes, which can be played rapidly with a smoothness that is hard to achieve on a guitar - this is the basis of the banjo sound in some styles of playing (e.g. melodic style). In other styles of playing the 5th string is sometimes (or usually) used as a drone string, and this is integral to other sorts of banjo sounds.
When I play guitar I miss the ease of playing smooth chromatic runs and such that can come so naturally on 5-string banjo. When I play banjo, I miss the bass notes and the relative ease with which you can have multiple things going on at once (e.g. in a well developed fingerstyle guitar arrangement).
Any instrument, whether guitar or banjo, is going to take a lot of time before you feel really comfortable and competent. When I first started banjo I played Foggy Mountain Breakdown (Scruggs style) and Cripple Creek (a frailing/clawhammer arrangement) over and over and over and over..........and over and over.......and over and over again, really badly, for about a year. Then I started adding some pieces, which I also played badly. About 2 years and many hours into this things really started to come together, and a lot of things I had been working on and playing badly for some time started to really sound good.
When I started playing fingerstyle guitar in earnest (after decades with banjo), I sounded pretty bad, and felt pretty clumsy for a few years. It was not until I had been at this for about 5 years that I really started to feel I was playing things really well (and I certainly was no beginner when it came to stringed instruments in general).
I don't think banjo is fundamentally any harder than fingerstyle/classical guitar. Due to the nature of the instrument, banjo players tend to play a lot in certain keys (G is "Gods Key" in standard tuning on 5-string banjo, though people also play a lot in C, and D, and there are a variety of alternate tunings that are commonly used, such as C, D, and D minor tuning). Standard tuning on 5-string banjo is an open G chord. Given the fact that a lot of pieces tend to be arranged in a few commonly used keys, and given the narrow neck, and the light strings that many banjo players prefer, the left hand tends to have it easier on 5-string banjo than on guitar. The bigger challenge is in training your right hand (but then when you go back to guitar your guitar playing friends will be amazed by your right hand!).
I see no advantage to playing mandolin first. Play mandolin if you want to play mandolin (I play mandolin too - it is much more like guitar flatpicking than banjo playing). If you are attracted to banjo (a sign of good taste, in my book
), then by all means get a banjo and give it a shot.
I learned with the aid of several main books:
The classic by Pete Seeger:
http://www.amazon.com/5-String-Banjo...bxgy_mov_img_b
Earl Scruggs' classic book:
http://www.amazon.com/Earl-Scruggs-5...9888678&sr=8-2
Tony Trischka's wonderful melodic banjo book (I used an earlier edition of this that I got in about 1980):
http://www.elderly.com/books/items/01-963149.htm
Peter Wernick's Bluegrass Banjo book:
http://www.amazon.com/Bluegrass-Banj.../dp/0825601487
Also, a big collection of banjo arrangements (that might interest you in a few years maybe) can be found here:
http://www.frettedinstrumentsnyc.com
(This is also a mother lode of fantastic fingerstyle guitar arrangements!)
I also had some Flatt and Scruggs records that I listened to a lot (and listened to slowed down to hear better what was going on - the Petticoat Junction theme song sounded pretty funny with the female voices (saying "Petticoat Junction") slowed down and sounding like big burly but rather effeminate men!).
I started out playing a Sears banjo from a Sears outlet store, and played that for about 5 or 6 years before building a much MUCH nicer banjo from a Stewart MacDonald kit. I can't stand to play that old Sears banjo now (but still have it). You don't need a top of the line banjo to learn on, but don't get a piece of junk (like my Sears banjo) either!
Your guitar knowledge will make learning banjo easier, and your guitar skills and sensibilities will enhance your banjo playing. Learning banjo will also enhance and transform what you do on guitar in some very nice ways.