Yes. These are for me the two big ones:
1-Amazing sustain/tracking.
And I really mean that.
Compared with the 33, when you play a sax or flute or organ
or big band sound, the PCM sustain sounds of the 55 makes your axe ‘breathe’.
It’s a beautiful thang!
2-Built-in modeling guitar processor.
Why?
One less thing to set-up and carry with you.
Personally I like my ‘Pod’ sounds better but the Roland modeled guitar sounds
can be very very nice and the 55 makes it all very convenient in one package.
I’m an old guy. So for me, the less stuff to carry, the better the gig. 
BTW
There are several threads on this board about MIDI guitar use
and I rarely participate because IMHO to effectively use a MIDI Guit
you have to have something that money cannot buy.
CHOPS! CHOPS! CHOPS! CHOPS! CHOPS! CHOPS! Did I say CHOPS?
MIDIguitar needs clean ‘MIDI Note On’ and ‘MIDI Note Off’ info to work.
Sloppy playing, while very human sounding (I always think of Keith Richards)
will not work. (very well)
On that note (ha) Keith and Steve Cropper are two of the best
rhythm guitarist I have ever heard. No ‘felt’.
Ironically there are two types of guitarist that have the
chops for MIDIguitar.
Classical players and jazz players and
those players do not usually want to sound like other instruments.
But K,
I do not want you to be discouraged by what I have posted.
Technology always moves ahead That’s how we got MIDI guitar in the first place.
It’s how you use it that counts.
Example:
When the first Roland MGuitar first came out back in the 80’s
(the one with the bar)
I saw a guy in a 3 piece metal band using it on stage.
He only used one sound but is was very effective.
Whenever the band play a ballad he had a string patch
layered under his power chords.
It was a beautiful early use of the Guit synth.
{‘-’}