Concentric butterfly valves are OK for utility service, these are soft seated or rubber lined valves. Generally disc is permanently in contact with seat.Eccentric butterfly valves (i.e. have an offset) have a cam effect, and so disc does not wear on seat through all rotation of disc. Triple offset designs eliminate vitually all seat wear, and are torque seated. They are the dog's danglies. Offset designs can be metal seated, and for these zero leakage can easily be obtained.
Traditionally butterfly valves are used in utility services (water, etc). This stigma is slowly being shaken off (supplier now call the high performance beasties rotary process valves)and now they are giving ball valves a run for there money in other applications.
For yours (water dist. mains) a rubber lined concentric would give average/good performance. A double offset valve would give superior performance, but is more expensive. Better (performance)still (and with the addtional cost implications!) is the triple offset, but I think unnecessary. these are now used widely in the North Sea for hydrocarbon service upto & including Class 600 (NPS 3 & above)for isolation.

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