What are the turn-off temps for the two switches?
If the 91 switch turns on at 91, and off at, say, 85; then you will have the fan turn on, but never cool down enough to turn off in the summer. So, it will be running all the time in the summer, even on the highway.

That being said, the 99 sounds like the better option - turns on at 99, which is certainly not too hot, then probably turns off around 95 or so. So, even in the summer, it should be able to cool down enough to switch off, particularly when on the highway. I can't remember for sure, but my low-speed electric fans on the SE turn on at approx 100, to assist the engine-driven fan, and they cycle off and on a bit when driving in town in warm weather.

One consideration, all cars using strictly electric fans have the fan tied in to the AC pressure switch, so the fan turns on to cool the AC condenser when in town. If not, the head pressure can go up pretty high (not good) and of course it won't cool well. Probably the hot condenser would pretty quickly heat the engine and turn on the fan, but it's an issue you would just have to check.

DG