As far as who is the SS or FS, these positions seem to be interchangeable in the NFL right now. Big bitters like Polamalu and Dawkins, after all, were technically free safeties. ESPN actually had a good article on this homogenization of the safety position, which I've sensed has been underway for the past decade: http://espn.go.com/nfl/story/_/page/hotread-safeties/nfl-shifts-big-physical-safety-more-versatile-model
As for the Eagles this is an interesting situation with Jaiquawn Jarrett starting this week... Both Coleman and Allen have played well in recent weeks after slow starts; Jarrett last week replaced Allen and played well vs the Bears, though that's obviously not a large enough sample size to draw conclusions.
If you think draft position translates into talent or potential, Allen and Jarrett were second round picks while Coleman snuck into the starting lineup as a 7th rounder. But Coleman played great vs the Bears and ever since the Redskins game really, and he looks like the best tackler on the team (and yes, I saw his missed tackle on Cruz).
Allen has had a major knee surgery and now has a concussion history, which may or not affect his aggressiveness in the future. Jarrett has shown the least of the three, but I have a hunch that he may start to blossom over the second half of the year, and you have to believe they drafted him in the hopes of starting by at least next season.
I feel like Coleman, while actually 2 inches and 10-15lbs smaller than Allen-- is a more consistent tackler and player overall who will hold on to his starting job.
I think Allen might be the odd man out here in the long run, which is a shame as I think he has potential as well. Maybe Juan can set up some kind of 3 safety semi-nickel package and put DRC on the bench. But don't count on it...