It is based on your last team.
I just set my first defensive team, I'm a little apprehensive as to whether or not it will work out. I normally drop a couple of hundred places throughout the day from 250-500ish so I'll test it with this new team.
My attack PVP is
Ahsoka
Talia
Old Daka
Luminara
Asajj
Defense is
Chewie
Savage
Luminara
Jedi Consular
Old Daka
I'd like to have another tank or a counter hero, but I'll test that out before I invest anything into a defensive team. Hopefully it will slow them down enough to run out of time. Three healers would be a massive pain the almost 2 tanks would be a mission. If I were facing this team I'd take out Old Daka then Jedi Consular then Luminara then Savage Opress then Chewie, hopefully that will be enough stalling.
I'll update in 18 hours.