I don't think they are similar, I mean in in terms of what they provide (creativity, goals assists) they are but they have different style, Miki is a strong runner, while being great dribbler, Mahrez is different level as for dribbling and he seems to play with his head up more too, more of playmaker when he's deeper and when he's close to the goal more of a selfish bastard who runs at people and score more goals, better finisher, Miki is more balanced, works harder but lacks a bit of flair on Mahrez, that's my assumption of the two, maybe Martial in the final third reminds of Mahrez more than anyone else in the team.
Would be a big shame if he let him go to any other team