Although you can do both, either 32-bit only or a combined, the preferred way to do it is as you say, a combined binary. I imagine that the differences will be fairly small because of the shared types and the resource sizes not changing. I'll have to experiment and see the results.

Just a note on how to build for 32 bit and still target IOS: You have to set the architecture for armv7, armv7s and then set your deployment target to 7. I have yet to deploy an app doing this so your mileage may vary.