Well, how does the mileage and maintenance compare? The only thing limiting the future mileage of these cars is the past mileage and the quality of care.

I'm guessing the B230F is newer and has fewer miles, but an engine swap is a lot of work. Your putting an engine that's probably only about a decade newer in the car--unless this new engine is a lot more powerful (I don't think it is) or is in a lot better shape, it's going to be a pain swapping them and getting all the proper components upgraded and working.

If you've got the mechanical know-how, sure, go for it (it sounds like you do with the transmission swap), but remember that this project is going to take probably twice as long as the transmission swap will take. It might be worth it to do everything at once, though, instead of having to swap things five or ten years down the road if the B21F does go. How much free time do you have?
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'89 244 GL -- 107,XXX miles (see profile for info on car)