It's an interesting question especially considering how young these two are. I'd say they are both blue-chip type goalie prospects but that doesn't always mean much. For every Tuukka Rask there's a Justin Pogge or two out there. The reality is you probably can't go wrong either way although ED does offer an interesting solution and I'm not inclined to disagree.
The big thing for me is that it will probably be years before you see either of these guys permanently in the NHL and potentially longer before we see them starting. Subban is going to receive the Rask treatment in Boston, meaning he'll play at least a couple of years in the AHL before making his way up and pushing for starts. Boston won't hand over the reins until they are certain he's up to the task, and he still has to overcome Rask at that point.
Gibson doesn't have a huge roadblock at the end road in Anaheim the way that Subban does in Boston but the Ducks do have a heavily stocked goaltending cupboard so he's going to be in tough to simply make his way through the system. At the NHL level you are probably looking at the two 30-year-olds - Hiller and Fast - holding down the NHL jobs for a while. Then you've got Andersen who is shredding it in his first North American season and Bobkov at the AHL level. Jeff Deslauriers is also in the way but as an upcoming free agent they probably let him walk.
There is a chance that Fasth fails at the NHL level and is let go so that simple internal promotion clears the path for Gibson to get an AHL job when he turns pro next season but if that doesn't happen then Gibson might be starting off his pro career in the ECHL. That's not a terrible fate as it says nothing of his talent and it's certainly better for him to be getting starts than to be toiling on the bench somewhere but it's hard not to look at the Ducks pipeline and not see how long we could be waiting for Gibson to arrive.
Long story short, you won't be seeing production out of either of these guys any time soon so maybe you want to look at who is going to fetch the higher price in trade.