Just do whatever you enjoy most. Don't grind. Play the game instead.
I love battle mode because it offers breaks from time to time (when you're dead). Gives me time to make a cup of tea, go to the toilet, talk to my gf, etc. The trick is to know when to take a break. ;)
Also, i love spectating cRPG. But that might just be me.
If you're looking for maximum XP, battle mode is probably best, because long streaks of x5 are much more likely on battle mode compared to siege. Don't have any data on this; it's just an observation.
Play siege for some quick and dirty action, but if you have the time, try battle mode for several maps in a row, do a bit of tactics and maybe switch your banner to a banner that's used a lot, so you'll get to play with the same players. Just a bit of tactics and the right banner will net you a lot more XP than siege mode will, i think...