Does Delenn have access to the White Star fleet? Is she still a member of the Grey Council and/or Rangers? If so, I'm gonna have to go with Delenn (as much as I love Guinan).

I mean, Guinan is theoretically super powerful (though those powers are left very vague and our only real hint is "Q had a scared"). Delenn is "we demonstrated it in universe" powerful. She more-or-less singlehandedly changed the fate of the universe. She stopped the Earth/Minbari war basically by herself, balanced out the prophecy of Valen, forced Earthforce to retreat from B5 by threatening to wipe them off the map, and led the Rangers (among other total badassery). She was part of a group that essentially kicked God out of the universe.

The other thing I'd point out is that Guinan's most demonstrated skill is her empathy and skill as a communicator. I'd lay money on Guinan having a better chance of not letting it get to the point where words fail because time after time, that's what she's really good at. Every time she plays a heroic role, it's because she finds a way to say the right thing to the right person at the right time (e.g., "If you don't go back to San Fransisco, we'll never meet," "You need to let Picard go," and "This ship is wrong, it's supposed to have children and families on it," come to mind.).

TL;DR: Guinan has a better chance of not letting the shit hit the fan, but when it does, I think Delenn stands a better chance of coming out on the other side.