I know this is way late, but I just installed Dawnguard + Hearthfire a couple days ago, and I'm curious about which side people went with. I chose vampire because I heard the Vampire Lord powers are badass, but honestly, I'm not enjoying it that much and I kind of wish I went with Dawnguard for this character.
I enjoy the sun and all, and the vampire's not being able to function properly in the sun is a huge con for me. I also don't like that you need to 'revert form' to use your vampy skills, it just feels inconvenient (the main reason why I never used my werewolf powers). And the whole feeding thing is also something I don't want to have to worry about. I'm seriously thinking about removing vampirism once I'm done the questline - is being a Vampire Lord the only 'reward' you get for siding with vamps?
I don't really know that much about the Dawnguard, but I hear the characters are more personable and likeable compared to the arrogant jerks in the Volkihar Castle. And they have cute doggies (and the armored trolls)! And crossbows. I already looted one from a Dawnguard, but what's so special about it?
Anyway, I guess I'm just wondering what the pros and cons of each side are, mostly, and which you prefer.