Lukaku and Martial both scored exactly 23 goals without penalties. Martial is not our penalty taker so it'd skew the data to include those in the comparison. Also, Lukaku played 3567 minutes while Martial only played 2922 minutes this season. That's a difference of seven full games, basically.
I'd also say - though this is not fully objective - that Lukaku played in a side that was stronger relative to its opponents in the league (82 points compared to United's 66, 81 goals scored compared to United's 66).
In conclusion, Martial has had the better season, he's also the better footballer and not a massive twat like Lukaku.