I hear that if you stir-fry vegetables, then frozen is a better option.

I think it depends on whether you can get your heat high enough.

The point of stir-frying frozen veggies is to brown the outside while not overcooking the inside. Normally this is done by cooking non-frozen veggies at very high heat but a regular house stove can't do it properly -- so a workaround is to use frozen.