Piano vs guitar

Piano because it’s better for composing about any music genre. But a guitar is more portable and possibly less expensive, so it has that going for it.

I chose guitar, because of rock and roll music, and the portability of a guitar. However, I don't actually own a guitar, but I did inherit my grandparents old piano.

If it specifically for studying music theory then definitely piano. Outside of learning music theory though either is a fine instrument.

I play a little guitar. I was learning classical guitar but stopped because I had to travel for six months and never took lessons again. I should have continued.

I like the piano.

You don't really need to play a specific instrument to learn music theory. Once you understand basic theory you can apply it to whichever instrument you prefer. I'd choose piano though.

Both, but I've been playing piano since I was young, so piano is 1st choice.