In order to have even the faintest chance of being a good violinist, you need excellent relative pitch. The violin has no frets to guide you, like the guitar does.
Plus, holding and fretting w. the left hand, and bowing a violin with the right (or vice versa for lefties) takes a different kind of physical stamina than does the guitar. Both arms are held in the air.
On balance, technically, the guitar is easier to become proficient in.
I studied classical guitar for a year and really enjoyed it. After my sixth week of violin lessons, I ( and my neck) knew that we were not meant for each other