A couple blips on the radar:
Rattay does nothing, nothing, spectacular and has a wet-noodle arm, but he embodies the efficient, game manager type that many coaches like in a backup. As a third string quarterback he's an asset and should cost next to nothing. The Hawks certainly can't go wrong by signing him.
Simms is the more interesting and therefore controversial prospect. He's currently the Buccaneers 4th string quarterback. The Bucs are hurting for wide receivers, with only 35 y/o Joey Galloway a sure thing. Simms was quantifiably good in 2005, but his struggle to unseat Major Applewhite in college gives me pause. Simms had a reputation as a choke artist in college, but, ironically, arrived on the NFL scene with a big comeback against the Washington Redskins in week 10 of the 2005 season. Simms could be thought of as Hasselbeck's longterm replacement and would likely only cost Seattle one of our superfluous wide receivers.
So what this comes down to is how you value Chris Simms future. I don't think he has much of one. As I said, it shouldn't cost Seattle too much to acquire Simms, but the potential downside is anchoring the franchise to a quarterback who just doesn't have a ton of ability. Meanwhile, Tim Rattay is essentially free and has proven to be a competent quarterback in multiple seasons and within multiple systems.
Another thing to factor in is the supremely talented crop of quarterback's in the 2008 draft: Colt Brennan, Brian Brohm, John David Booty, Erik Ainge and Chad Henne are all first round quality QBs.
So, in a nutshell: Simms is the HOFer's son, the prototypical 6-4 signal caller who has never earned much respect with his on-field performance. The Hawks will have to give up something more than salary to sign Simms and may even commit an extended contract in the belief that he's the Hawks' future. Tim Rattay is the misfit. A player who has produced every chance he's had, but never impressed with his weak arm and slight/fragile build. Rattay plays for the Hawks as long or as short as Seattle feels. Signing him commits the Hawks to finding a longterm solution in the offseason, quite probably in the draft.