Crota has specific drops for each section on different difficulties. Unlike VoG which does them by sections but normal/hard does not matter. Take for instance the fusion rifle from crota. You can only earn it on normal. I think only primaries can be earned on hard.

yeah only primaries on hard. secondarys and heavys only pop up on normal. on VoG the difference is hard gives you all the normal drops but adds hard drops to the loot table. so if you want the best chance of a normal loot table drop (like VoC) you run once on normal (as you get one roll on the smaller loot table and so have a higher chance of getting VoC) then once on hard. but on hard you could get for example the helm or Vex so you've less chance of getting VoC. the Raid mode you should run depends on what you want and how desperate you are to get that item. are you happy to run the raid on normal then hard for a slightly higher chance at a drop than just doing hard once.