if you want higher ilvl and artifact advantage in pvp, then go 99. if you want more things to do such as random dungeon queue, order hall missions, some legion professions (im not sure if there are other things 99s cant do), then go 101. you will still have a slight ilvl advantage in pvp if you have full ilvl 835 to 850 boes, but the gain is not as high as 99s