Steel type is the first thing worth comparing: high-carbon and laminated Japanese blades take a keener edge but need hand washing, while stamped stainless holds up to daily dishwasher cycles.
Look at blade length against your board and your hand size rather than against the photograph, and check whether the listing states the hardness on the Rockwell scale and whether the edge is symmetric or single-bevel. Sets are worth it only when you would actually use every piece in them.