Obviously Nagato had the rennigan from a young age and he used all his life to master rennin... on d other hand, Tobi has had it for days or weeks in manga time scale... he was able to control paths and Bijuus
If u paid attention to what Kakashi was was saying at d beginning of the fight with Tobi; Kakashi mentioned that Tobi couldn't master the rennigan total because its such a tough toll learning how to control all the Bijuus at d same time, and even in that Tobi didn't master the bijuu control properly because they were revoting against him. This was because he had no time, d war was already starting so he tentatively left out d other paths of d rennigan.
also, someone mentioned that Nagato crippled when he used Gedo mazo, thats because he had never summoned it before, and he was still young, where as all Tobi had to do was sync himself to it rather than summon Gedo from d scratch. So with few evidences, its hard to say who is better in terms of the space of time they have had to master d rennigan but over all without adding the time factor, Nagato is clearly better and has a vast knowledge about d rennigan since he's had it for all his life. it will be biased to say Tobi is greater.
If Tobi was better at using rennigan, he would have taken it off Nagato since d early stages and accomplished all his plans but instead he left it with him and trusted him to master it and just sat back to manipulate Nagato. Even Madara placed his hope on Nagato and Tobi wanted Nagato to use d 7th path for him alone, he cld hv done it himself if he was better.... simple
