Buy the best you can afford. Metal impellers are worse than plastic in the end if the bearing goes bad down the line. If you search there has been a few threads on this.
I'm on my third water pump in six years I've owned my '99. The first was OEM and went for 82K miles (CarFax showed no wp service had been done), the second was third party and went for 2,400 miles and the third is OEM and has about 20K miles. I'll never buy a third party pump again.