Peggy, from my own experience, the keyboard was easier to learn. The keyboard I had, also had the built in rhythm tracks, and you made the chords with 1 or 2 fingers. 1 finger for major chords, and 2 fingers for minor chords. It was easy to make your chord changes and make them on the correct beat. NOTE HOWEVER it did sound a little "cheesy". But I just used it to write the song, not perform it.
The guitar wasn't really that much harder to learn, but learning to keep rhythm and making chord changes on the correct beat was harder, and still is, for me. I will tell you, that learning guitar, your fingers will hurt. But, the "bleeding fingers" is a cliché that guitar players use to tell students that's when you are really learning. Your fingers don't bleed. But the tips do hurt until they get some callouses. THEN, as you learn more things on the guitar, your whole hand may even hurt sometimes from using new muscles. But that too goes away after enough practice.
Now, let me add, this goes back to the discussion we had on being a lyricist. It isn't the instrument you use to write your songs; It's the rhythm you keep while doing it. So, since a keyboard, even the cheaper ones, will probably have built in rhythm tracks, you will be able to use that rhythm to write your lyrics.
These days, I usually write a new song using my guitar, but I still use a rhythm track on my keyboard to make sure the lyrics and chord changes fit the way I want them too. And, I still use the rhythm track to practice songs to make sure I'm keeping rhythm. BTW, after years of this, I pretty much know all the chords I need and can change fairly easily, BUT I still have a problem with keeping rhythm sometimes, and THAT is more important. Especially when you are performing live, and can't go back and correct your mistakes!
phil g.