stayed in both quite a few times - both have been recently renovated, sky last year, olümpia a bit earlier. at least in my case, both have offered breakfast
both are walking distance from old town, sky closer to both old town and the ferry terminals. google maps gives the distances 1.4km (sky) and 1.8km (olümpia)
i'm concierge, both always give me upgrades, olümpia either superior or business class, sky a suite every time.
in my opinion, sky is much nicer. olümpia business class rooms are nice, standard and superior pretty small (18sqm vs 30sqm standard rooms at sky).