We Know Hashirama has Earth and Water techniques (due to Wood) however we don't know of S-T ninjutsu. He has Wood release mastered to an extent where we can't even use Yamato's skills as a fair comparison. He fought and defeated Madara, who at the time had EMS and the Kyuubi, and he has the ability to take control of several Bijuu. Although he can't actually use the Bijuu in a battle like this that ability is rather irrelivant.
Tobirama has unsurpassed water techniques (could be argued that Kisame could match his) so his Water techniques would surpass his brothers. Tobirama has S-T ninjutsu, Sword of TG (unless you want to exclude fillers), and Edo Tensai (to an extent, uncompleted but still probably usable, seeing as the Kage that commented on it, recognize the jutsu so it can be assumed that it was usable to a certain level and further improved by Oro and Kabuto).
Really we're going based on hype for both as we've only seen them fight as Edo summons and not at full power. We've only heard of what happened with Hashirama vs. Madara. We've heard of less of Tobi. I personally like Tobirama more and he has a enough skill to at least critically harm his brother if not kill him, and the same can be said the other way around for Hashirama.
As it stands we can't come to a fair conclusion however I bet most will say Hashirama because his hype is higher and most see him as the strongest Hokage and never have I seen a thread where one discused Tobirama as a possibility for the strongest Hokage. He's usually fourth on the Hokage lists I see so most are going to go with Hashirama but it can't be concluded and a lot of favoritism is going to play into this.