Butterfly valves close in 1/4 turn, gate valves don't. Butterfly valves are less expensive.
Butterfly valves are smaller.
Butterfly valves indicate valve position by handle position. gate valves (with the exception of OS&Y) dcn't.
Butterfly valves will have tighter longer lasting shut-off for either a liquid or a gas. With soft seats you can get bubble tight shut-off, and even upwards of ANSI Class VI with metal seats. You can NOT get these types of shut-off's with gates. You can get a gear operator that gives them multi-turn operation like a gate, even though they are a quarter turn valve. Butterfly's are also much smaller and lighter - on large valves makes them much easier to install. Also, they are a bit better at modulating the flow vs. a gate which should be strictly used for on/off applications.
However, the disc of the Butterfly will be in the flow path. They could erode depending on the material passing through them. And, for the high end valves, they can be more expensive first cost.
I would not use a gate valve on something that does not open/close at least once a year. Gate valves tend to get stuck (molecular wear) in the position they are left in. After say 2 years, you try to close an open valve, well, chances are, it wouldn't, it could pop on you.
gates are good for throttling duty in my opinion, but definitely not for positive shut-off.
I am more partial to full port ball valves for shut-off, even globe valves on by-pass applications.
