The Leafs would probably point out that Connolly, other than 3 years, has been a fringe NHLer or worse his entire career, and was not even good enough to crack a rebuilding team's roster, while Mrazek just had one really down year (mostly hampered by numerous injuries) but has had a solid track record of success the rest of his career.
Connolly is guaranteed dead money. Mrazek is not. Leafs would rather have the useful asset than the dead money, so unless Chicago includes some kind of sweetener or the Leafs find a third team to dump Connolly to, I don't think we consider it.