I believe a good way of comparing ODI allrounders is giving equal weightage to batting average, batting strike rate, bowling average and bowling economic rate. No era adjustments required here as well. Extreme examples like averaging 5 at SR of 200, if they exist are not going to help the player concerned one bit. So if we create a metric called allrounder proficiency factor(APF) i.e (batting average*batting strike rate)/(bowling average*bowling economic rate), anyone above APF of 1 is an above average allrounder.
Kapil ends up with an APF of 1.33. Klusener has a much higher APF of 1.58.
Interestingly though, between the exact age that Klusener debuted(i.e 24 years and 4 months), and retired (i.e 33 years 0 months), if we look into Kapil's record, he has an APF of 1.63, making him a moderate upgrade over Lance. Note that this is not a conveniently picked period.
Having watched Kapil Dev in action, I would certainly say that he was a pale shadow of his former self from 1991 or so in ODIs. Likewise for Klusener in 2004, but he retired before going into steep decline.
I rate Lance Klusener highly and do not have a problem with anyone rating him over Kapil Dev. Just that I believe Kapil is really in the discussion for the best ODI allrounder ever.