Both are top pvp races for priest for each faction. Human prob is better since alliance is stronger in pvp now, but you can play undead and be as much successful as human. All depends on which race looks better for you
Ive been nightelf male shadowpriest 2200+ rating 5v5 in WOTLK. Shadowmeld is good and helpful in world pvp/ defending objectives bg’s. It’s just that i have a trauma of ud rogues