There's another solution if you don't plan on going out in the cold, then decide to do so later: get the low-r-value pad you're looking at now. Then, if you decide to take a trip when you might get into lower temperatures (or colder ground) than it will handle, buy a closed cell pad like the Thermarest RidgeRest Classic- about $15 in the 48" length. You put it on top of your inflatable pad. You'd only need the short length to keep you warm from knees to shoulders, most likely.

A single pad with a higher r-value will weigh less and be a more efficient way to get to the same place than two pads, but if you only plan to go out into the cold rarely, two pads would be a practical solution.