I say sm, but with the curse mark being senjutsu based(ironically only mentioned when sasuke needed senjutsu from juugo) they're 2 sides of the same coin no?
SM. Curse mark is basically an incomplete form of SM. the only real advantage it gives over SM is that it can be activated much faster, but it still has the downside of slowly killing the user.
Sage Mode is better because of it's benefits but in terms of strength their chakra is the same. It's very possible though that with training that the Curse Mark could become equal to Sage Mode.
CM is derived from the principle of SM so obviously sage mode as it actually means controlling nature energy whereas CM simply absorbs it and amplifies what you already have. no extra add ons