imo keep and it's not close. If you mull to four, how much better a hand can you hope for? Especially with the new mulligan rules, five lands is not abysmal. I remember watching one of Ben Stark's videos and he said that with the new mulligan rule, he'd keep virtually every six-card hand. I suspect he exaggerated somewhat - a 0-lander I'd still mulligan - but I'd also keep a 6-land hand.
EDIT: Finally had enough gold for another draft. Went 4-3. Disappointing overall. I thought I had a pretty good deck but still lost three games. First one was perhaps to a play error although I was already rather behind (next time, do NOT trade just before
The Eldest Reborn goes off!). Second game opponent had a really good deck. He played Elfhame Druid on turn 2. If I had drawn my Shivan Fire I'd have killed it, but I didn't. Then he kicks Saproling Migration followed by Verix Bladewing. Dealing with the two halves of Verix Bladewing takes up all my removal, and then opponent lands Song of Freyalise for the win.
The third loss was disappointing too. Opponent was playing a Grizzly Bear aggro deck on the play. Meanwhile my earliest play was on turn 3, with a ramp spell. Turn 4 Baloth Gorger was bigger than his creatures he still attacked to bring me to four health. Then I mostly stabilized. He had a Slimefoot which I killed, but he had the black Memorial in play to recur it. I really needed to draw my Fight with Fire, but didn't. During the final two turns of the game he first draws Thallid Omnivore followed by the saproling lord, while I drew two lands. His final attack was just enough for lethal. RIP. I guess I benefited from opponents making play errors (one opponent clearly didn't realize that Jousting Lance provides first strike), but 4-3 is still disappointing.