ARW wrote:

You need to make sure that the minimum load on the transformers is met
(usually this is 20VA)

While on about GU10s and MR16s ... is either one looking favourite to be
the long term choice for LED, or are both likely to exist for ages?

Obviously with GU10 LEDs you've not got to have transformers/drivers,
but are the drivers crammed into the base likely to overheat and fail
quickly? MR16s obviously need a separate driver but that can live
anywhere above the ceiling ... and be replaced separately from the lamps
if necessary.