What you're basically saying is that it's better to finish 1st grade math than to get half way through calculus.
Kakashi only had to learn to use 3 abilities, only 1 of which took any skill beyond natural ability, whereas the Rinnegan has 7 unique paths, each with individual abilities, and allows the potential to learn any jutsu in the world.
All that is said is that the Rinnegan's power is even greater and can corrode, possess , and reduce the vitality of the user. Handling the Rinnegan is more difficult, I can agree, but there's really no difference noted in the skill level required to wield the Dojutsu. Kakashi mastered his borrowed power whereas Nagato could not, therefore, Kakashi was better. Rinnegan is inherently stronger, but comes at a price which is the aforementioned repercussions imparted on the user.
Listing out everything is superficial. Nagato was predisposed to wield the Rinnegan, Kakashi wasn't with the Sharingan. Kakashi overcame this inherent disadvantage and mastered the Sharingan while Nagato couldn't. Frankly, since the manga and databook emphasizes Kakashi's proficiency with the Dojutsu, we're led to believe that Kakashi's feat is more impressive than Nagato's who was only remarked for his predisposition to handling the Rinnegan.