There's not much I can add which hasn't been said above, almost a year ago I was trying to find a logical conclusion to the same question, I don't think there is one.
Mine was answered in the same way as yours seems to have been answered, pure chance by what becomes available.
If your looking for an age related plate then a whole donor is probably the best way to go but at the end of the day are you building the car for the car or the plate.
I believe the MX5 gear is more obtainable and a little lighter but there are less completed builds so possibly not quite as much support as for the ford running gear.
I'm not sure on how many diff ratio options are available on the MX5.
I know all these questions rattled round my head for a while, and it can be a daunting aspect when in the first stages of thought/planning etc.
In the end for me, I somehow fell in love with the BMW V8, I stumbled on some Scorpio Cosworth running gear on the bay, I've still got to sort a gearbox and decide on whether to use the ford/BMW diff, but gradually its getting there.
At the end of the day this is your build, your creation, your going to invest a lot of hours/money/stress in it so see what comes up and if it fits with what your dream is (within reason) then go for it.
Good luck
