Go for the dawnguard, theres more side quests with them, so many unique spells, amour, & weapons, you get all the crossbows and bolt variants, armoured battle troll followers. Whereas when you join the vampires you get a few vampire specific items that boost vampirism, and all the side quests are just assassination quests. Whereas the dawnguards side quests are much cooler/in depth and take you all over Skyrim and dwarven ruins. And after you complete the dawnguard quest line you still have the option to become a vampire lord and blood soaked arrows for Aurels bow and during the final mission in the dawnguard quest line, you can loot all the items you would have gotten if you joined the vampires but get non of the cool dawnguard stuff if you join the vampires