I would pick based on what sounds fun to you. I'd definitely have frost as one of your specs. It's a very durable spec for mages and can pack a punch.
As for the other spec, you have arcane, fire, or another frost spec. Odds are that the frost spec you have no will be good enough for anything you want to do with frost, so I'd say don't choose frost as the second spec. (I play a warlock and both of my specs are in the same tree.) This leaves fire and arcane.
I would say you should at least try out fire for a bit. I think around level 50 it can be pretty fun. Fire is just really different from the other specs. You have aoe abilities and certain casts that you can do while running around in circles. At level 69 you gain an ability called Living Bomb. This ability does damage over time for a few seconds. When the time is up, the target you put Living Bomb on will explode and deal damage those around it. You can put 3 living bombs up at once. This combined with your aoe and running around blasting things can cause some explosive damage.
To give you an idea of what I was able to do with it, right around that level of 69... most people were doing about 100,000 to 200,000 damage at best in battlegrounds. As a frost spec which is very strong in pvp at that level, I could easily reach 300,000 damage. I tried out fire spec for a bit and was able to do 450,000-600,000 damage.
If you can't get the hang of fire, then I'd try out arcane. Arcane is much simpler, but I feel like it's also a bit less exciting. It can pump out some nice damage on a single target, without much skill or thought.