We've seen enough of Hiruzen to get a feel for how strong he was. Tobirama is vaguer. On feats Naruto is ahead, but then we just saw Tobirama do something virtually no other character (bar Hashi and Madara) could do- snap Edo Tensei like a twig. If it wasn't for Oro's recent power up, he'd have broken it, and ditto if Kabuto had summoned him. That, and other context, makes me feel we're likely to be seeing some Tobirama feats soon that may put him above Naruto.
Technically on my top 20 list I rank Naruto ahead btw. I don't think we can be sure he's ahead though, which is the point I was making.
