I have a couple of metronomes, both by Korg:
The MA-30: small and easy to carry in your violin case. I find it difficult to use without sound (on mute) since it has some type of digital pendulum that is not easy to view unless you put the metronome on your music stand. In addition, the buttons are small and you find yourself clicking a bit everywhere to get what you want. Nevertheless, it's still a great carry-on tool.
The KDM-2: large metronome with a powerful sound (loud beat), a visible light on top and multiple options. I like it since it's easy to use and you don't need to press a zillion button to change the tempo. I also find it helpful when playing with other musicians (e.g. a quartet), where everyone is able to hear the beat to adjust to the tempo. Smaller metronomes have a tiny speaker that can barely be heard by everyone.