Dawnguard, because you aren't force to kill npcs in town with their radiant quests.

The dawnguard,they have cookies and what do you get when you join the vampires??human flesh and a bite to the neck

I have a good feeling it will.
Lets just hope so and stay optimistic.

Both sides have many flaws.

Depends on your own character.

Both the same really. Depends on your perspective of the situation. I didn't like Harkon's idea so I joined Dawnguard. Until I soon realized, the Vampires questline was the same thing.

In the end, it doesn't matter because it's the same questline (other than the Blood Chalice). It's more of a choice on whether you want to be a vampire lord or not. I just joined the Dawnguard because I didn't want to be a vampire. Nothing against vampires.

Funniest thing I've heard all day.

OP: Both are awesome and different (gameplay-mechanics-wise, not quest-wise), I'd recommend trying them both but start with the Dawnguard.

Again, everything the dawnguard can do really the vampires can do and vice versa almost.

Vampires can go around with spells bows swords and crossbows (assuming you can keep finding a supply of bolts) and the dawnguard can do the same thing and become a VL later.

If you want both vampire lord and crossbows I'd recommend going dawnguard, learning how to craft bolts and crossbows, and then becoming a vampire lord, then you have pretty much the main gameplay additions at hand at the same time xD

Go Dawnguard. You get Vampire Lord and the crossbow : )

The dialogue is drastically different depending on which side you chose.