Whether Charlie Crist serves as governor or U.S. Senator really isn't that important. He hasn't exactly covered himself with glory as a leader in Tallahassee, and he's likely to be pretty lackluster in Washington as well.
There is a sense that he's cutting and running (to use a Republican turn of phrase) just when the going is toughest...maybe anybody else would do the same, under the circumstances.
Senators aren't term-limited, and Floridians tend to reelect their senatorial incumbents for life just because they recognize their name on the ballot.
Then, there's the talk about his possible run for the presidency. To me, he doesn't seem to carry the mass for that kind of job. Maybe the new Mrs. Crist, who is rumored to have aspirations to be First Lady, will provide him with the gravitas and "fire in the belly" to lunge for the brass ring.
In any case, he'll look smooth and debonair in a tux at all those Washington parties. That counts for a lot in the corridors of power.