which would be easier to learn how to play?

Well, if they're like any other instrument, they're both equally difficult. Playing the piano and playing the trumpet are equally difficult, even though they're completely different instruments in the way they're played. A trumpet can be too hard for some, as you have to have very tough lips and have very good control over your lips to play it well, where the piano, you just have to have decent hands, but the physical requirement to play each instrument is of equal toll to each body part. Get what I'm saying?

A rhythm guitar (I don't really know anything at all about guitars) is the one in the main melody, right? That means the rhythm guitar would have to learn how to lead and keep the melody in pace, whereas the base guitar has an equally tough job keeping the harmony, making the rhythm guitar sound better. Both are equally difficult and equally important, as far as I know.