I'd say Minato is stronger, but not in raw power, jutsu, or chakra, but rather because of his FTG technique and rasengan. Raw power, jutsu, chakra, and chakra control are means to win, but intelligence is vital along with power to win. If Minato and Hashirama fought 1 vs 1, it would be no easy fight. It would be a tough, long drawn battle between the two Hokages, I'd say it'll be pretty even since Minato can use FTG with his kunai to avoid attacks, but Hashirama would be no slouch either. Both would learn each other's techniques and try to counter them. What gives Hashirama the edge in a higher chance of winning is his medical ninjutsu which requires no handseals. Honestly, the battle could go either way, but I believe Minato would have a somewhat larger chance of winning. If Hashirama did lose to Minato, no doubt he would be very proud for someone to have surpassed him to protect the village Hashirama founded. I highly doubt he would be happy if no Hokage could surpass him.