That's a toughie. Both of these are in the same section as meadow sage (S. pratensis, sect. Plethiosphace) and the nemerosas. Regeliana can be confused with virgata and amplexicaulis more commonly. I think transsylvanica will be a taller plant.

Transsylvanica grew well for me in South Jersey. I would still have it but for the fact that it didn't like crowding. Thompson-Morgan used to sell the seed and may still do so.