I perceive that MECARD is somewhat more common for this purpose. It is
certainly the format defined by DoCoMo, who pioneered all this in
Japan a decade ago. I agree, outside barcode-land, vCard is more
common. MECARD is never used except in barcodes.
I don't have any good data on which readers support what. I perceive
that few readers do anything with contact data, since that is not
their focus, and, because really only iPhone and Android let you add
contacts anyhow. (Not sure about Symbian/Nokia). For instance the J2ME
clients out there couldn't do something with contacts if they wanted
Anyway our reader reads both and is probably the most common 2D reader
on Android, so you're OK either way there.
I suppose I would recommend MECARD since I perceive it has slightly
more support, and is more compact, and has more history as the way to
encode contact info in barcodes. That is to say I would be very
surprised to find a reader that reads vCard but not MECARD.