Big fan of siege here, but mostly because there is no cav. Used to be a big battle fan, but when I tried siege for the first time, it was heavenly not having HAs, Cavaliers XD, or respawn times. Both modes require a certain amount of tactics, the problem with siege mode and brains, is when very good players resort to the scoreboard, as opposed to objectives (like flags, gatehouses, etc). Personally, I think siege is MUCH more difficult to tailor the match with a group of freinds (unless you have 5 huscarl shielders). A nice part about battle mode though, is being able to go make some food if you die early [ :.
In siege, you are in motion 24/7. Attacking consists of: suicidal people charging up a ramp, while smart players find a secret entrance and open a door for them, if the doors stay open, attackers win. Defending consists of: 1/4 players are dead yelling FALG LAFG FLAG! 1/4 doing useful things, 2/4 fighting on walls.
My experience in battle has been: Bah, freakin cav killed me. GAH, I hate xbows! OUCH, well atleast it was thier best player. Sometimes you get that amazing 6 kill streak in 10 seconds, but a large amount of my deaths in battle consist of "now where the hell did that come from?" Which makes me sad until I respawn.
Also, siege is more freindly for lighter armored players imo...less focus fire. In battle you better be neo if you are not wearing some shiny stuff (or have a shield).