1 MD = ~10 INT, so the ilvl 55 with 59 magic damage and 15 int = 605 equivalent INT. The ilvl 70 with 17 INT and 57 = 587 equivalent INT. So in this case, the ilvl 55 one is better. Is the ilvl 55 one HQ or blue and the 70 is green/NQ or something?

This is a rough estimate. Current stat weights are based on level 60 and current gear. Generally, weapon damage is 10 (+/-2) of main stat.