I can agree with trading Thomas over Rask, but not just because of his age. If you trade Thomas in the off-season and give Rask the starter's position, the Bruins save $3.75M on the cap, some cap space that they could use to improve the team in other areas.
The Bruins just got Kaberle, and finally have a legitimate top four of Chara, Seidenberg, Boychuk and Kaberle. No more Ference or Kampfer masquerading as top four defensemen. The $3.75M cap savings from trading Thomas and keeping Rask could definitely go a long way in re-signing Kaberle.
Not to mention, packaging Thomas with a player like Paille and maybe some other assets could bring back a pretty decent top six winger as well. Fills a hole the Bruins will probably have next year since Recchi could very well be gone, likewise Ryder, unless he's willing to take a drastic paycut.
Deal from your strength to shore up your weaknesses.
Granted, the age argument does have to come into consideration at some point. Rask is much, much younger and is one of the top young goaltenders in the league. He can shoulder the load in years to come.