Warning: Cricket blogging!
A superficial analysis of the numbers might suggest that Murali has the edge. But then you need to remember that Murali's figures are padded by the fruits of no fewer than 25 tests against hapless Zimbabwe and Bangladesh (Warne has a total of three tests against such feeble competition). Secondly, Wanre's record away from home is just as good as his record at home; Murali is less effective away from the comforts of Sri Lankan wickets. Thirdly, their records when they confront the world's best players of spin in India are broadly similar. You can make an eminently defensible case for either man.
Still, the current series against Australia is of considerable importance to Murali: in his previous three tests in Australia he averages 63 with the ball. (It behoves one to lament that, so far, the retirements of Warne and McGrath have not, irritatingly, slowed the Australians. Not yet anyway.)
But the question of whether Warne is a better bowler than Murali or vice-versa is the wrong one. Of course, it's the only one you can really ask if you want to have a debate since if you were to wonder which of them is the greater bowler there is only once credible, sensible answer: Shane Keith Warne.
Numbers play a part in establishing greatness of course, but they can't tell the full story. There are players whose statistical records struggle to bear witness to the full extent of their greatness (Keith Miller is one obvious example that comes to mind, even though his figures are pretty damn impressive in themselves). Numbers aren't enough.
Warne is without doubt the greatest and most important cricketer of my lifetime. Though the art of leg-spin had been kept alive by Abdul Quadir and Mushtaq Ahmed in Pakistan and by a number of journeyman Australians, it took Shane Warne's genius to show that leg-spin - the greatest and most romantic form of spin there is - was no mere frippery or luxury but that it could be a deadly match-winning weapon even in less than perfect conditions.
Warne made spin possible again, demonstrating the absurdity of the attitude - especially prevalent when England were skippered by dullards such as Bob Willis and Grahame Gooch - that spin bowling was too risky, too unpredictable, too idiosyncratic to be relied upon. A fast bowler could be smashed for a couple of boundaries and it was nothing to be worried about; a spinner suffering such a fate would immediately b hauled out of the attack. That thinking led to the drudgery of picking four identical fast medium bowlers all the time.
Warne's freakish command and control raised him above all those who would emulate him of course, but his efforts dismantled this dreary, anti-spin mentality. (Granted, some of it still lingers, at least in England, where until Monty Panesar's emergence spin bowlers were expected to justify their selection by means of their usefulness as a batman; a criteria never ever imposed upon quick bowlers, no matter their own shortcomings with ball in hand). Warne changed the way we thought about the game and changed it for the better. In his own way, then, he was a revolutionary.
And, of course, there is the other matter that no-one really wants to talk about: the unfortunate fact that Murali, through no fault of his own (thanks to that defective arm) is a chucker. It's a sad business, made worse by the ICC's refusal to do anything about it, but I don't quite see how you can talk about these two bowlers' respective merits without mentioning that it requires a charitable interpretation of the laws to call one of them a bowler at all. What Murali does with the ball is remarkable but, IMHO, it merits a mental asterisk.
Mind you, I might have to revise this opinion in the unlikely event that Murali can take the Aussies down a peg or two this month...
UPDATE: Commenter Matt chides me for not pointing out that though Warne has fewer matches against NZ and Bangladesh, he has played more tests than Murali. True, which is why I think it is possible to build a statistical case supporting Murali. But that's rather the point isn't it? The numbers are a useful but insufficient pointer to greatness.
As for wickets-per-test figures, how do you factor in the fact that Sri Lanka have generally speaking relied upon Murali whereas Warne had to compete for wickets against Glenn McGrath et al? Alternatively you might say that batsmen rarely had a breather against Australia, ensuring that they always felt pressure - and were hence liable to crack at any time - whereas Murali had to carry the Sri Lankans all the time (no disrespect to Chaminda Vass, mind you). I suspect that's a wash.