I would pbly do the extent - if you SAN is healthy and solid, then really the only risks are administrative: someone later will not know that LUN1 and LUN1A function together to provide the space for datastore-lun1
I've heard others also carp about extents and the problems they are - but I've not experienced any of these problems. I have a feeling this is one of those old "back in the day" adages that ... really in this situation does not apply.