It is really just a difference in flowering or size, they both originate from the same Genus and Species.

So, is there really no difference in hardiness?
Is snow queen considered showier? When I look at pictures, I have a hard time discerning the differences.

According to one site:

Some improvements make this Oakleaf Hydrangea significantly better. Its upright habit is useful in small spaces; its rich green leaves take full sun and its large, white, well-filled flower heads are held on rigid stems and do not droop after rain.

My experience in zone 6 is that they tend to be more likely to die over the winter when they are young. If you can get them through several winters, they seem perfectly hardy, at least in my zone. They never made it through the winter if they were fall planted.