Which instrument is easier to learn with a background of music: cello or violin?

So for the school I'm going to this year we have to choose an instrument to learn throughout the school year. Our choices are the viola, the violin, the cello, or the upright b. I went into a music store and tested them all out and out of the four my favorite two are the cello and violin. I've been playing guitar for 6 years and understand music, how to read it, and how to create it. I already make my own songs for my band and have a good background of music. So which do you think would be the easiest to learn if I already know music? The cello or violin? Or if you think the upright b or viola would be easier to learn than those two, please recommend them. It would be best to get answers from people who play both of the instruments. Thanks!