What should be understand before hand before making any post in a vs thread is that the logic of absolute limits does not work in Naruto. Meaning that if A>B and B>C does not imply that A>C. Who wins a fight as more to do with match ups than it has to do with standard quantitative of power, hence why using the maximums of data book to compare shinobi is stupid unless we're talking about a 20 to 40 or something like that, and even this means very little.
Now to answer your question, Nagato has de ability to absorb chakra and use it as if it was his own. In that sense Bijuu Mode is an enormous pool of chakra from where Nagato would be able to drink, unlike Sage Mode who's chakra would turn Nagato into stone. Yet it is plain obvious that BM is far stronger than SM.
So the question here would lay over two extremely important questions: "Is Nagato fast enough for BM Naruto?" and "Can Nagato absorb a BM TBB?" We truly have no way to answer these questions so to be honest to say Nagato>BM Naruto is just as much speculation than to say BM Naruto>Nagato.
On the other hand if we were talking about Pain and not Nagato, BM Naruto would make soup out of the 6 Paths. And it should be note that having BM does not take Naruto's Sage Mode away. In fact we've seen that Naruto is now able to have a Shadow clone in Sage mode while maintaing BM which mean he can probably have more clones gathering Natural chakra while he is running BM. And that would mean he could probably maintain a fairly constant stream of SM Shadow clones. Who would be very dangerous for Nagato.
But all and all I think it's a pointless question or affirmation as we really have not enough info to answer.