why do replacement thermostats come in two temperatures (170 and 180 degrees)?

Climate. If you live in a place that gets very hot and never gets cold, South Arizona for example you want the thermostat to open and begin coolant flow sooner than if you live in the NE and need a little more time and to keep your engine warm, from running to cool. Running too cool is hard on an engine. They have a wider range than that also, 165 to 190. New(er) cars should use strictly what the manufacturer specifies. Also, once the car is at operating temp it has NO difference in cooling capability

I got curious and thought to myself, self, you don't know that much about the 'big picture' about thermostats and opening temperatures ranges. So I researched. The thermostat sets a minimum, not a maximum temperature of your car's operating temp. You don't want too cool, I knew I was right about an engine running too cool being stressful. So a lower range will open sooner, and cause a longer warm-up time. A car needs to run hotter than 160°F. So if it's 40° below 0, the thermostat will keep your engine hot enough by partially closing. Sometimes you might see big-rigs with a canvas cover, with a zipper. But even with a 160°, the engine will still warm up past that and be fine. It sets no maximum. But needs to open before optimal running temp, if the car is designed to run at, say, 200°, and the 'stat doesn't open until 200°, it might get too hot before coolant has a chance to circulate. NO thermostat is strictly not good, either the coolant does not have time to dissipate heat because it's flowing to fast = overheating, OR the same too-fast, or more volume can keep an engine from warming up. Usually the former. Bottom line: use what manufacturer specifies

Personally I would usually get the higher temp myself for warm up purposes.

I agree JP. Since it has nothing to do with max.