My previous Hunter Rain Clik lasted something like 5 years. Of course the problem is once the battery is done, the sensor is done. Meanwhile, Hunter redesigned just enough that it would require replacing the whole sensor and rewiring rather than just replacing the transmitter.
I haven't had the WR2 long enough to know, but specs say 4 or more years battery life under normal conditions. It uses common button cell batteries. I just don't want to have to replace and re-wire the whole thing after a few years as I did with the Hunter. The Rainbird just seems like a higher quality product all around.
The only thing the Hunter had going for it (which may or may not actually be an advantage) is that the sensor trips almost immediately when rain starts. The Rainbird takes a few minutes to trip. The drawback of Hunter's approach is that you may quit watering when there's not much more than a sprinkle. With Rainbird, you may water for a few minutes unnecessarily, but you won't skip a schedule when there hasn't been enough rain to make a difference to the grass.