Either should work for you just fine. My setup fluctuates and goes higher than 85, but your range is definitely good. If you had to choose, I'd go with 85 which is pretty much a perfect temp IMO....soil temp.
I have a thermostat for the heat mat and the probe is stuck in the center cell of the tray. It reads more of the center of the soil rather than the surface temp. The surface I would assume to be a little cooler, but never checked it. Maybe better to stick to 77F for surface temp if you have bottom heat in order to make sure the seeds don't get too hot.