NH lists the same 3 temp options on their website. I would install the hottest t-stat available. That is the best for engine life and fuel economy. In the summer, the engine will not run any cooler with a colder thermostat IF the engine is working hard and the cooling system is marginal.
In other words, if the engine is running at 200 degrees, replacing the 188 t-stat with a 168 will make no difference.