Would you recommend the violin or cello as a first (well second) instrument, if they had trouble with guitar?

When I had to learn them both in college, I found the violin to be more difficult, mainly due to the smaller gaps between pitches, and what was for me at least, more difficult vibrato. That, however, is why I would actually recommend that you DO study violin. If it's not your cup of tea, I think it would be easier to switch to cello from violin than vice versa.