I think it's hard to compare, yes Hazard plays in a better team but playing in a better team that's expected to win also means other teams set up differently than they would against a team like Leicester. Hazard is often highlighted too in most managers team talks as a focal point/target to nullify on top of the ultra defensive team tactics. With Chelsea this season and under Mourinho the whole team seemed unhappy with the tactics, he's alluded to the type of football he likes himself. It's also easier to look good in smaller team than it is to be playing in a top4 side, the champions league and so on where the expectations are so high and still stand out for years as Hazard has done.
To be fair Mahrez hasn't had the opportunity to pass that test yet, so you can hardly hold it against him. I think Mahrez is an unknown quantity in that respect but he'll certainly strengthen our attack either way. Hazard on the other hand would almost certainly improve us and IMO would possibly push us above the likes of Real, Barca and Bayern... although they might also strengthen too this summer so it's perhaps a tad too soon to call it.
The problem is securing Hazard will come at great difficulty and even greater cost. Chelsea might well be happily and deliberately wasting our time, so we either miss our other targets pursuing Hazard or even just to drive the prices up as the longer it goes on the more competition arrives.