I would think that both of their teams would prefer to retain them rather than trade them. I'm guessing that they would want to keep them as a veteran backup, or vet to start the season until the rookie is ready to start.
Cassel is due to make $2,650,000 in base salary plus another $3,000,000 in roster bonus, which from what I have read is all guaranteed; I don't think the Pats want to (or are in a position to) take on that contract with their cap situation. If Minnesota jettisons Cassel then they are back to Christian Ponder as their top backup, and that's a scenario that they clearly would prefer to avoid. The Vikings won only five games in 2013, with three coming with Cassel starting and one more when he played the majority of the game in relief of Ponder. The Vikings have over $10,000,000 in cap space, so I would think that they would keep Cassel and cut Ponder.
Hoyer's deal is for much less - $1,000,000 in base salary and a $250,000 roster bonus - but what makes that appealing to the Patriots also makes it appealing to the Browns to keep him around for the 2014 season. Unlike Minnesota, the Browns do currently have other options at quarterback, but none of them are as desirable as Hoyer. Vince Young hasn't taken a snap since 2011, and deservedly so. Tyler Thigpen has completed six passes since 2010 and was completely out of football last year; his 1-11 career record as an NFL starter says he is nothing more than a camp arm.
While trading Mallet away and adding Hoyer make sense for the Patriots, I don't see the incentive for the Browns to trade Hoyer (or for the Vikings to trade Cassel).
