It's no secret that I'm a big fan of Mullan, but Genge has really impressed me. Definitely raw, but no more than Marler and Mako were when they were brought into the EPS.
If Mako is out for the 6N then I'd have Marler first choice, Genge bench, Mullan in the squad (Mullan would come straight in to start in the event of Marler being injured - as much as I like Genge I wouldn't want him starting at this stage of his career - I think Genge offers more from the bench than Matt, and it'd get him 5/10/15/20mins and ease him into test rugby over the series).
As much as people might want Genge to leapfrog Marler you can't understate experience at the top level, especially in the front row - Genge might be better than Marler down the line, but I definitely wouldn't have him starting at this stage of his career. Genge is still very very young for a prop, he's only had 10 starts at the top level, Marler's got 4.5x as many England caps as that.