The X15 T.N is supposed to be much more corrosion resistant.
The 154CM, while stainless, is more prone to corrosion, but will hold an edge longer. I've had a number of 154CM blades and live 5 miles from the beach. I've never had a corrosion problem.
If you wanted a river or swamp knife, the X15 TN steel would likely be better.
Aside from that usage, for a pocket knife I'd heartily recommend the 154CM.