This may be a dumb question, but why bother debating longevity at all?
Forget "Tendulkar played for an extra 5 years" or "Lara played McGrath better" or whatever the debates are, and look at the overall stats for the common period of time they were playing, to compare their relative quality against each other.
Tendulkar isn't better by virtue of scoring more runs off McGrath, nor is Lara better because he dominated Murali (examples, not statistical fact). It doesn't matter who they were against, it was whether they could get the job done for their team.
Looking at the time they both played International cricket simultaneously,
the stats look like this.
Tendulkar has the edge in average (however he does have more not outs - make of it what you will), but both have the same number of centuries.
Lara's average per innings: 51.52
Tendulkar's: 50.67
Ostensibly, this analysis isn't perfect either - who knows how many more runs they would have scored before they got out, but managing the tail to eke every run out is a skill in itself.
Anyway, we can throw stats around however much we want, but when it boils down to it, its purely subjective. Personally, I rate them as equals - which is more or less what the stats tend to show too. They had different styles, but both were equally good at getting the job done over the same period of time.
Longevity =/= a better player IMO.
On a completely unrelated note, those stats show just how prolific Marcus Trescothick was - the first man to debut post-2000 to make the list, ahead of Sanga, Sarwan, Gayle and Sehwag
