For all the reasons you mentioned

Littleton had almost as many tackles this year as Fowler has in his career, not to mention the INT's and his 3.5 sacks.

Fowler's sacks are nice, but they are a glory stat in many ways when you figure that was only 12 tackles over the course of the season. He got 2.5 in the last game vs the 49ers and see how that turned out. His pressures would certainly be missed.

Littleton is a special player, Fowler is a god pass rusher.
In the unlikely event we can keep both, which would be nice, you have to go with Littleton. I would suspect that he might be a hair more affordable as well.