Down side to the "under the seat" location is that these things get HOT and there is little cooling air circulation under there. I would expect the excess heat to reduce the service life of the compressor in that location - especially if you use it for airing your tires back up.
I put mine under the drivers (left) side wing mounted to the top of the wheel well. That area is getting crowded now - preoiler, coil, headlight and driving light relays, and the ARB compressor. It's been there for 4 years with no problems