TL;DR:For low-skill comping, piano is
much easier than guitar.
I play and have taught both.
Learning chord shapes on the guitar is made more difficult because the B string is tuned a major 3rd above the G string, instead of being tuned to a major fourth. That obscures the guitar chord shapes. But if you take that into account, you can see the these are all the same shape. For example, look at the fingering of an E major chord:
E: 0 2 2 1 0 0
If you move each finger to the next highest string (up a fourth), and duplicate the note on the high E string to the low E string, you
should get an A chord:
A: 0 0 2 2 1 0
Not an A major chord 
But it's
not an A major chord... until you adjust the note on the B string a half step up
A:0 0 2 2
2 0 0
You can see this pattern as you move through the circle of fourths:
E: 0 2 2 1 0 0
A: 0 0 2 2
2 0
D: 2 0 0 2
3 2
G: 3 2 0 0
3 3
C: 3 3 2 0
1 3
F: 1 3 3 2
1 1
At the point where you get to the F chord, you can see that it's just an E chord moved up one fret.
These are what are referred to as "cowboy chords", and the C and G are typically simplified a bit to require less fingers.
Once you see how the E, A, D, G and C chords are related, you can apply that pattern to the minor chords, and so on.
However, even knowing about the pattern, they still have to be learned by rote.
In contrast, the underlying scales that make up piano chords are a bit more obscured. Going the sharp way round the circle of fifths, the altered note is the major 7th of the scale. That is, C major has no sharps:
C: C D E F G A B
G: G A B C D E
F#D: D E F# G A B
C#A A B C# D E F#
G#E E F# G# A B C#
D#There's a pattern to the addition of sharps and flats, but it's not immediately obvious how it applies to chord shapes. So learning the chord shapes on the piano tends to be purely rote memorization. Fortunately, similar shapes can be grouped together:
C, F, G major: white/white/white
A, D, E major: white/black/white
Ab, Db, Eb: black/white/black
Once you know the fingering for a piano chord, it's relatively easy to transform it a variant (minor, suspended, major 7th,
etc.).
When I teach piano, I'll start out teaching accompaniment first, because it's fairly easy to learn and puts people on par with beginning guitarists.
Although I'll
show a student the root chord voicing, I'll have them
learn the voicing in the right hand with the fingers between F (below middle C) and E (above middle C). Think of it as keeping the piano in "cowboy chord" position. It's easy to move around from chord to chord without having to thing of inversions (just like a beginning guitarist) and keeps the chords in a nice register. All the left hand has to do is play the root of the chord.
For example, instead of voicing a C chord in root position as [C E G] it'll be voiced as [G E C] with the left hand playing a C note.
This approach is basically playing chords on the piano the same way a guitarist strums on the guitar.
But unlike the guitar, there's no awkward stretching of fingers, it's easy to move from one chord to the next, and it's less painful than fretting strings. It's easy to learn "complicated" chords such as Fmaj7 by treating them as Am/F. You can do the same thing on guitar, but it
sounds a lot better on piano.
