Look, taking wickets and averages are directly proportional to the quality of the attacks.
If you are the lone great horseman, you will take more wickets than you would if you had backup from other great bowlers. Simply because the majority of wicket-taking balls come from you. But being the lone great horseman, the opposition has a much better chance of seeing you off ( while attacking other lesser bowlers) than if you were in a great bowling attack.
A classic example would be the aussie attack vs the sl attack. Australia has two great bowlers- McGrath and Warne, who often wreck the opposition. Gillespie is/was excellent and Lee, while being an ordinary test bowler, is definately better than anything SL has gotto offer barring Vaas and Murali.
As such, Warne has more competition for his wickets which is why he gets less wickets/match. But owing to the fact that OZ bowling maintains a much tighter pressure than the SL one( since OZ has more quality bowlers and as such, you cannot play off one quality bowler and wait for a mediocre one) the pressure on the batsmen is stronger.
Since the batsmen dont have the luxury of playing you out of the attack, you should have an excellent average. This is classic of the WI pace quartets- you simply COULDNT play the 'good bowlers' off and wait for mediocres to pop in the crease- you play off Marshall and Holding and you get Garner and Roberts waiting for their chance. You play them off and Holding and Macco are back in the attack. Its not like with Hadlee or Imran, where you could play them off and capitalise on lesser bowlers.
This is the sole reason ( IMO) why Hadlee and Imran average more than Marshall/Holding/Garner etc.
But considering the fact that Murali takes more wickets than Warne and averages significantly lesser - lesser even IF you leave out ZIM/BD, along with the fact that Murali is far more successful against the best players of spin- India- than Warne, it isnt surprising that many consider Murali as a superior bowler.
True, Murali has more favourable pitch conditions, but Warne has a much much better bowling attack at his disposal and favourable pitch conditions are overrated.
Simply because one adapts to the conditions one faces the most often.
In the subcontinent, you need balls that shoot off the pitch and subtle variations of flight to snag wickets - which is the forte of Kumble and which is why Kumble is deadly in the subcontinent. Great spinners from outside the subcontinent - such as Gibbs, Laker, Warne, etc. typically rely on exgaggerated bounce and bigger turn to snag wickets- and they dont do that well in the subcontinent.
Same with pace bowlers - the non subcontinental pace bowlers have typically relied on bounce and short pitched deliveries to take wickets while the subcontinental pacers typically pitch it up far more often and rely on sheer speed and/or movement to snag wickets. This is adaptation- outside the subcontinent the pitches are bouncier and jucier, giving the bowlers an obvious pattern of attack. Short pitched ones dont work in the subcontinent, simply because the bounce is not as exgaggerated. On the other hand, the abrasive pitches in the subcontinent helps reverse the ball better and cut it better too.
Which is why non-subcontinental pacers typically do worse in the subcontinent while subcontinental pacers do worse outside the subcontinent.
Owing to this, i think if positions were switched- Warne in SL attack and Murali in OZ attack, you would see Warne pick up more wickets- 5+ wickets/match but his average would balloon up by a few points. And you'd see Murali drop to under 5 wickets/match but his average shrink by a few points ( putting him in the below 20s zone).
Most lone-horsemen bowlers show this pattern when compared to one who has support.
Hadlee took more wickets/match but had a worse average than Marshall.
Donald's average improved markedly when pollock showed up ( before Pollock he averaged 25-ish, after pollock he averaged 21 ish) while his wickets/match didnt change significantly.
The fact that Murali takes more wickets and averages more than Warne, as well as doing significantly better against the best players of spin clinches it for Murali as far as i am concerned.
Warne is a more brainy bowler than Murali is IMO - he is better at sniffing out a player's weakness and constructing a plan to bowl to that.
But Murali is a more skilled bowler than Warne - he has more control than Warne ( which is why he is hit around the park far less often than Warne is) and his bag of variations are far more lethal.
Both are world champions and i would love to have both in the attack but i think Murali edges Warne by a little.