I would recommend using a f2p account that is attached to your main p2p account so you can still have access to your mounts and companions, yet you will always have the ability to play the free account even if you dont pay anymore.
Rogue is what I would recommend out of the classes you listed, since they are great for both WSG and AB, and also arena etc.
Arms and all warrior specs in general are pretty boring to play, the have few abilities and very little nuance involved at this level IMO.
Ret has very weak damage atm but is a pretty strong off healer so in a sense they are currently a totally support oriented class, as opposed to burst oriented. They are supposedly getting a buff next patch.
Due to the fluid nature of WoW where class power balance is constantly shifting, I'd just go with what you like at the moment. Level 20 twinks arent a huge time committment so you can always make a few different classes.