i know about chakra density...i mean anything else...
I can agree that chakra density alone isn't enough, but considering both share vast similarities and Oodama Rasengans are partially a lot more powerful than Rasengans due to its increased chakra density, we can confirm chakra density does play a role. In general, we should also compare Bijuudamas to Oodama Rasengans. In general, Bijuudamas can blow up a mountain whereas an explosion created by Oodama Rasengans cannot do that. Furthermore, 50%/100% Kurama's Bijuudamas are much stronger than that and should deal more damage overall.
Therefore, if we add in the FRS component to both attacks, the one that naturally deals more damage on its lonesome (Bijuudama) is clearly going to do more damage even with the FRS component added to it.
Hope that helps somewhat.