I would prefer alfalfa for these reasons:
higher proteien
more palatable, meaning she will eat it more readily
Less roughage, meaning less of a haybelly on show heifer
less chance of accidiosis as she will eat more alfalfa hay in combination with her finishing feed in comparison to oat hay with her finishing feed.
if she was due to calve soon, I would go with the oats hay as it seems to help preventing milk fever especially n dairy heifers.
if you can't get alfalfa hay, oats will be fine, but given a choice, i'd go with the alfalfa.