For mechanical arrangements, and a fuller sond, piano. For emotional fluxuation and portability, guitar.
Versatility, practicality, portability, degree of cultural entrenchment, guitar almost encompasses an entire category of different instruments in and of itself. It's equally at home in almost any genre of music; all much more so than piano. I would still vote the latter for no objective reason. Piano music has a sophistication and elegance that I love. I value its greater 7+ octave range, and polyphonic advantage allowing for more complex harmonies, I value that more than I do the guitar's greater versatility in tone, dynamics, ability to bend/slide notes, harmonics, far greater range of techniques strumming/picking/fingerstyle etc. Then all the gear, amps, pedals etc. that make it a world of possibilities in comparison. Ultimately though its the pure, clean, full, rich tones of piano music that do it more for me... all this with the qualifier that I'm comparing them as solo instruments. Within the context of a band, it swings as far the other direction for me. Also the music I always gravitated to in playing the guitar was almost more suited for piano- jazz chord melodies, ragtime, classical, big band all at once, almost all exclusively fingerstyle, generally playing chords and melodies simultaneously. Most of that stuff breathes better on piano. And although I spent only a fraction of time on the piano in comparison, and was far more proficient on guitar, I felt like I understood the piano as an instrument, and its music better. Re: guitar + piano = keytar, maybe also guitar + piano = harp, could be better. Similar range, and music read the same, but pluck strings directly vs. hammered.
Much easier to make a living playing the piano than the guitar (or any other instrument). I've always envied really good pianists. They have so many more options to make money than basically any other instrumentalist or vocalist. A really good/versatile pianist is worth his weight in gold. That being said, it takes MANY more hours of monotonous repetitive practice just to become proficient. So they deserve it.
Guitars, but I'm biased, I'm a guitarist. I love all instruments, but if I had to choose a second it would be drums over piano.
To be fair, we're talking about 1 Elton John. I can think of a ton of different contemporary bands with guitarists -- and I'm guessing they don't have issues getting laid. I'm sure there are plenty of people playing piano as well, but uh...
