Even though the ODI average differs by only about one run (and you might even dispute that on averages as openers), Jayasuriya is miles better at one day cricket. Put simply (before I go into convoluted detail), Jayasuriya is more likely to make a match winning score.
Think about it. Put two imaginary opening batsmen in a 5 match ODI series. One gets out in the 40s twice and out in the high 30s three times, averaging in the 40s for the series. The other scores a century, two 50s and two ducks; he averages about the same. However, the second player may actually have won the series by providing the team with big platforms.
Sehwag is much, much more likely to lose his head and miss out on a century, something that can make him a big liability if he's not producing scores. Jayasuriya in fact has one of the better 50-100 conversion rates in ODIs. Although his hundreds percentage isn't up there with Tendulkar or Gayle or Ponting or Saeed Anwar, it is better than 6%, significantly better than Sehwag's 8 centuries in 170 starts.
Of course, if we turn to just opener's stats, Sehwag's 8 hundreds come in 140 starts, 5%. However, Jayasuriya's 25 hundreds also all came from opening, so in 340 innings that is 7.35%. They may seem like small numbers, but the guy with a seemingly untouchable 41 hundreds, Tendulkar, still only converts 1 in 10 innings into a century. In truth, you might say it is only small things that are the difference between the greats and the could-have-beens.
In Tests, though it seems Sehwag is the clear master, somewhat ironically, since his Test game is comparable to (or better than) many top ODI players. However, where Jayasuriya's weaker patches have been largely attributable to injury and lack of match fitness, Sehwag genuinely seems to be a victim of his own success. He averages 91 against Pakistan and I think therein is an answer. His biggest scores have all come against Pakistan, often on some of the most appallingly flat tracks to have been used in Test cricket. Beyond the
1st Test of the frustrating 05-06 tour of Pakistan, it seems Sehwag finally popped.
Maybe he has lost total faith in his methods; that there is now a little voice saying "Dravid wins more games" (well, if the voice was honest, it would say Dravid loses less - only one Dravid hundred has been in an Indian loss, but that's another story). Whatever the case, Sehwag's loss of form is among the most dramatic seen in world cricket. Perhaps the drop was the worst way to deal with it. Perhaps they are trying to hard to deal with it.
Then again, perhaps there is reason for Sehwag to doubt his style. Going back to Jayasuriya's now closed account, 7 of his 14 centuries were a part of wins for Sri Lanka, while only 2 of Sehwag's 12 have had positive results for India.
I suppose we can question the value of averages and statistics quite a bit, but when it comes down to what results in wins, whatever that may be, it's very hard to doubt such a quality.