I like Lopez as the 26th man, but Lukes has been impressive and probably deserves a bench role as well.
I could see them going with Lukes initially since Lopez has options, and there would still be enough defensive flexibility that the roster wouldn't suffer, but it's really hard to say the Lopez wouldn't be the optimal 26th man given that he can play anywhere with average to above average D, runs well, and might run into a HR every month or so. Lukes for certain has the power edge over Lopez but even then it's probably below average power at the MLB level, but ... with only being an OF, when there's already ... 4 capable CF and 1 corner guy on the roster and he's not better defensively than any of them except probably Biggio... he'd basically be a pinch hitter/runner and never really see the field except in blowouts.
I'm on team Lopez, and I wish Lukes the best of luck in grabbing a bench role elsewhere if he ends up getting DFA'd