I just realized that the internal pressure sensors for four tires don't include the spare (some of us think glacially slowly!).
So if I ever have to use the spare, I give up continuous pressure monitoring with an internal system, unless I buy an extra one or two sensors. With external sensors, I'd just move one to the former spare.
I would expect that only internal sensors would provide accurate temperature measurements, with poor thermal conduction along even metal stems coupled with effective air cooling. Pressure measurements should be equally accurate with either type. I think that pressure alone should provide most of the benefit; temperature might make a difference with slow leaks that result in the tire running hotter, which increases the pressure and might prevent the pressure sensor from alarming while driving; once the tire cools off, the pressure will drop.
I noticed that TST recommends only using metal valve stems with its external sensors. I sliced a candy bar to give me a 0.5 oz. weight of similar weight and size to the sensors; it is pretty light and I would expect rubber valve stems to survive a long time, but they will take a significant extra beating. Metal stems should provide higher reliability, especially on rough roads which seem more common when we're towing.
A related question: does anyone have a method for rotating tires including the spare. Otherwise, I can see the possibility of needing to replace a new and unused spare tire due only to its age.
Fore-aft swaps would be simple, but the spare wouldn't be involved.