Physically, 448397-B21/452372-001 == MHGH28-X[T|S]C
Firmware feature wise, there appears to be a single difference.
All 3 of my HP MHGH28 (2 rev X1, 1 rev A3) came with DPDP enabled.
While on Mellanox MHGH28, only rev >= A4 appear to have it enabled.
DPDP = ability to run IB on port1 + 10GbE on port2 simultaneously.
So I'd say the HP cards have more functionality out of the box.