batsheva, I had similar experience when switching the bikes (and my bikes have different wheel sizes and different gear ranges). I suggest you to look at its "gain ration", proposed by Sheldon Brown, which takes wheel & crank sizes into account. As GLC1968 said, the crank size also matters.

Gain ratio = the wheel radius / crank length * (front teeth / rear teeth)

Don't worry about the math. Sheldon has put up an online calculator for that.

Create two tables of gain ratio for your two bikes and compare them side by side.

In my experience, assuming the tire resistance coefficient is the same*, I tend to choose the "gears" which give me the same gain ratio on the different bikes for similar cadence. Simply, that's my output power I can produce without straining myself.

Hope this can answer your question.

*PS: For road-bike tires, this is a fair assumption. But when comparing bikes with very different kinds of tires, this would affect on your choice of gears. Gain ratio is purely mechanical and doesn't take the resistance into account.