this is why i wish they would implement a "legendary loot spec" i want to farm Mythic+ dps gear without the chance of it dropping a legendary im only going to use as an offspec. also 2 days remaining until i can equip 2 legendaries with zero drops so far...
personal loot started the same time LFR did (i think), it gives more loot per mob than Master loot and can be swapped between players that were there for the kill. You can get identical loot, i got portalguard handgrips off the same boss 2 weeks in a row (one was warforged). I'm not sure the deciding factors other than loot spec, i dont know how much "is this an upgrade" comes into play since otherwise people would try to game the system by putting a piece of shitty gear in the gear slot they wanted to improve.