Originally Posted by
noglider
With a seatstay mount, you will be adjusting it, as the light will be bumped and moved around the stay. If you're the type to check things often, that could be OK.
Mine's been on my seatstay for a long time and I don't think I've ever adjusted it or had it bumped. OTOH, the one on the seat tube below the bag does get bumped sometimes when I rummage around for something in the bag. The lower position on the seat stay is also good for not shining so directly at a rider immediately behind while still being plenty bright for an approaching motorist.