The question is not whether or not Sestak would make a good SecNav. The question is, did someone in the White House offer Sestak the position of SecNav if he withdrew from the race for Specter's seat. If they did, and Sestak has given every indication this is the case, that would be bribery and that would be a felony.
I don't think it would be bribery at all. If the white house was seriously considering Sestak for SECNAV - and I see no reason why they WOULDN'T - why in the world would they want him to knock the incumbent senator out of the running only to NOT take the senate job himself?
That's just it, isn't it? They didn't want Specter knocked out of the race. What better way to insure Specter's seniority and continued support than to remove the competition? It's a bonus that Sestak is more than qualified for the SecNav spot. The question, based on your post, now changes: Were they seriously considering Sestak for SecNav prior to his run for Specter's seat? If not, why the offer after his decision to run? If he was not in consideration prior to the race, the offer, in my mind, is tantamount to bribery.
Of course, unless or until Sestak opens up and spills all, none of us will ever know for sure, and all this discussion is speculation.