Mmmmmm.... I'm thinking... It is possible choose two of them?
I think each one have their pros and cons; so, analize by points.
1. Time of game play: In the case of battle frontier there are many things to do, because the number of the factories, with their own theme; meanwhile the battle subway has a more limited options. Point for battle frontier.
2. Recognicion. In the battle frontier once you beat one time at the leader of a factory, you obtain a silver medal, but if you beat for second time you obtain a gold medal in your card, but in the battle subway there aren't this. Point for battle frontier
3. Surprise factor. In the battle frontier you have a better idea of what you will face, but in the battle subway the randoms are more exciting, because you choose your 3 ideal pokemon to see if they are more equilibrate. Point for battle subway.
4. Competitive training. Although the battle frontier its good, i guess the battle subway its a better way to have and idea of the competitive pokemon game, because the difficult increase in each encounter, and are very easy. Point for battle subway
Conclusion: I like both of them in the same proportion, so i want to see this two implements together, but if i must choose one, i go with the battle frontier, because i like the medals XD