For those big battles, you need to join a bigger clan and be member for some time as they need to know you and your skills.
Otherwise, are you in a clan? Best way to get xp, is of course rolling a server. Battle or siege doesn't even matter here but either way are good to get lots of xp when playing with many clan mates.
If you play without, I'd say siege is the better game mode to get good xp. I hope you're also a melee player as it is usually the melee player to decide a round, or better saying he rather decides a round, in making more kills and stuff. Of course there are also archers on the battlefield who do many kills and turn a fight easily around, however strong melee players probably got more influence there.
Anyway, I say siege because it is often depending on the map. There are maps that are "easier" to be defended or attacked. Battle does of course also have maps that are better for cavalry for example, however I experienced that when playing without a clan, you will mostly get x1 - x3 on battle, but will more often roll x3-x5 on siege.
Can't say much to DTV.