Surprised Hewitt is winning this poll. It is worth noting not only did he only ever reach 2 French quarterfinals, but he was badly beaten in both. Once by Gaudio, and once by Ferrero. This spells things out even more clearly to me. I highly doubt Murray would have beaten either of those, but he would have been more competitive (in his best years at RG) and against Gaudio atleast gotten a set.
Murray was actually competitive in his 2011 semifinal with Nadal, despite it being straight sets. 2014 he was blitzed, however this is Nadal, not Ferrero or freaking Gaudio. Neither had any actual "good" wins at Roland Garros, so no difference there, other than the fact Murray was in the semis twice.
Other than his small titles on clay (Murray never plays small clay court tournaments or would have some too) the only argument I could see for Hewitt is their matches vs Ferrer. A past his prime Hewitt took Ferrer to 5 sets at Roland Garros once. Prime Murray is owned by Ferrer on clay (while owning their personal rivalry on every other surface), and clearly been the inferior clay courter. However I am not sure if this says much. Hewitt might just be an uncomfortable matchup for Ferrer, and it is only 1 match anyway.