What is the difference, in level of technical difficulty, between a 5
string banjo and 6 string banjo?

Which one would be best for a beginner (although my assumption would be a 5 string)?

Hi telarium,

It's difficult to be precise about the level of difficulty, because it
depends on many variables.  One could say a six-string banjo is 20%
more difficult to play than a five-string banjo, because it has one
more string.  However, one of the strings on a five-string banjo is a
high drone string that is not normally fretted, so if you don't count
that one, you could say the six-string banjo is 50% more difficult. 
On the other hand, even though you normally don't fret the drone
string with your left hand, you still have to deal with plucking it
with your right hand, so if we count the five-string banjo as having
four and a half strings worth of difficulty, the six-string is 33%
harder.

A six-string banjo is tuned like a guitar, so the player's familiarity
with guitar playing will make a big difference in the relative
difficulty.  A guitar player will find the six-string banjo much
easier, but since it doesn't have the high drone string, it will be
difficult to make it sound like a five-string banjo.

It also depends on the playing style.  The usual way of playing a
five-string banjo is finger-picking, where individual fingers pluck
individual strings.  This requires some practice.  You could play a
six-string banjo this way, or you could just strum it, which would be
significantly easier.

As myoarin noted in the comments, standard banjos have four or five
strings.  The four-string banjo is used in traditional jazz, and the
five-string banjo is used in American folk, country, and bluegrass
music.  The six-string banjo is relatively rarely used--its main
attraction is that it enables a guitar player to get banjo sounds
without learning a whole new instrument.

In my opinion, a five-string banjo would be more suitable for a
beginner, especially one who is interested in bluegrass, like you.

I hope this answer is helpful.  If you need any more information,
please ask for a clarification.

--efn

I believe a 6 string banjo is usually played in standard tuning(like a
guitar)although it could also be played in open tuning (which a 5
string usually is).The way the instrument is tuned may be more of a
determining factor in how easy it is to play,it is much easier for a
beginner to learn to play in open tuning on either instrument.I would
suggest using a 6 string if you intend on playing traditional jazz and
a 5 string for bluegrass.You could experiment with tunings until you
find one that's eaiest for you.

Fantastic - 

I plan on playing some bluegrass, so this helps quite a bit.

Thanks!

Here's some beginners tips
http://www.folkofthewood.com/page4841.htm 
http://www.jbott.com/begin.html

I may be a bit old fashioned  - can't remember the last time I saw a
banjo close up, but I could swear that the classical banjo only has
four strings and the 5-stringed one was a later development.  The
first page of sites on the subject seemed to support this.
Hey, but that should just make it easier to learn to play.