Quite honestly I go with the highest flow rate for outdoor spigots that I can find as that has nothing to do with conservation.
One thing to remember is that the flow rate is dictated by water pressure, the size of the pipes, the size of the hose (e.g. a commercial style hose will deliver more water than one of those super skinny hoses that curls back on itself), etc...
In order to conserve water, I prefer to:
- use spray nozzles,
- water early in the day or late at night so it won't evaporate,
- only use what the plants and ground call for, and
- use mulch to help hold the water in.
While not related directly to conservation, I also make sure that I turn the water off when I am done and do not rely on the nozzle itself.