Franson did seem to have top pairing ( or at least #3) upside during the playoffs. He didn't look too bad despite being given heavy minutes. On the other hand, Gardiner, was used in a more offensive-oriented manner ( ala Letang, Karlsson) and succeeded.
Gardiner does seem to be a better fit for Phaneuf than Franson IMO. He can skate around the ice effortlessly and is very creative. We're going to be retaining Phaneuf so it would make a lot more sense to build around him. Gards has that slight edge despite having much lower production and, arguably, upside.
Regardless, I'd keep both for now. I don't think their values are at the high point right now. Franson is just getting into the game due to missing the camp. Gardiner, on the other hand, is starting to play a puck rushing game. To make things worse, we've lost 3C, and therefore the environment they're in is quite compromised. To expect higher production from either may be too much.