How many will get their pvp primarily from arena, and how many from battlegrounds?

Arena is a straight 5 vs 5.
Battlegrounds are mutable, with 5 vs 2, 5 vs 5, 5 vs 11, etc.

Sure we can compare one team vs another, in a straight 5 vs 5.
Which is more akin to arena.
Ideally, a team would be good at that level of play.

But how does each team do, at say Blacksmith in Arathi defending (or assaulting) against 10 random players as a 5-man unit without support from the rest of the BG?
Which composition is better in that situation?