Which creatures from the alternative upgrade options do you choose?

It makes sense to take the alternate upgrade for unicorns only if the hero has a developed light magic skill and the Blessing spell.

Actually, no. If light magic is developed, then their ability isn't really needed, as most of the spells are area-of-effect. And the light magic skill will most likely already be at the Expert level. +2 to the parameters won't give a significant advantage, and besides, the new unicorns have not only more hit points but also more damage (9-24, compared to 10-20 for the old ones).