That's nonsense. Orochimaru doesn't use the Curse Seal on his own body and he nearly became a Sage. The only reason he couldn't is because of all the bodies he kept stealing, none of them were strong enough to become Sages. You forget that not every human can become a Sage. They need to have a naturally large chakra. Jiraiya had a large chakra. Naruto has an enormous chakra.
Likewise, drawing in energy isn't the only thing it takes to enter Sage Mode. One has to focus the natural energy with their own chakra. This is what Kabuto learned from the great Snake Sage.
Kabuto implements Juugo's DNA because it's far easier to absorb natural energy when your body does it automatically. Naruto and Jiraiya both find themselves in situations where they can't absorb natural energy unless they have toads or other ninja watching their backs. Kabuto has no such limitation thanks to Juugo's DNA.
Oh yes there is a way to compare.
http://naruto.wikia.com/wiki/Sage_Mode
Naruto is considered a perfect Toad Sage because of the red eyeshadow. Kabuto is considered a perfect Snake Sage because of the dragon transformation. Inorder to see which is superior, you look at what they both inherently gain with their transformations aswell as both their inherent weaknesses.
Naruto has a time limit with his Sage Mode and runs out after two FRS's, which means he has to sit still and absorb more energy. Kabuto has Juugo's ability continuously absorbing natural energy for him, meaning he can move at will and never run out of natural energy.
Jiraiya could move at will and still gather energy, but ONLY when Fukusaku and Shima was fused to him. And Naruto currently can't fuse with either of them thanks to Kurama.
I will admit however that the only reason Kabuto's SM is superior is because of experimentation/Juugo's DNA. Doesn't change the fact that it's better though.
How the hell does Manda being evil prove that EVERY snake is evil? I never said snakes were good, I simply said that not all were inherently evil. That means that the only ones that can be considered evil are the ones that have been shown to be evil. Which is ONE snake: Manda. You haven't the slightest chance of proving otherwise either.
As for Manda's modification, he was DEAD. Originally he was a normal snake. The boss summon of the snake pact, but still just a snake. Then Sasuke got him killed, but Kabuto revived him and did experiments on him to alter him (gave him limbs, increased his sensing abilities,etc).