question about maps and playing in team

1. What is the advantage of playing quality maps? like 6% quality or 20% quality? i know higher level maps = ilvl of items is higher but dunno what the quality do

2. Isn't teamplay hurting my "item find" if i play by myself all items are mine and if i do this with other players the items are split so i could probably lose much items yes?

Quality of maps increases quantity of items dropped in a 1:1 ratio.

Party play offers a higher degree of safety while increasing overall drops and exp at the cost of individual loot and exp. In a 6 man party, you get something like 66% loot and exp (just a random number, can't remember), but you do clear faster and you technically get more overall loot per map if playing with friends. Party size does not affect map drop rates.