I want to start the dawnguard dlc but I'm still having trouble on what side to choose. Are there any exclusive items only for one side and who do you guys recommend I should join

There's exclusive items on both sides as well as the accompanying side quests to retrieve some of those items. As to which side is better to join, I personally prefer the Dawnguard, but it really depends on what you're looking for. If morality matters at all to you, there's no tricks. The Dawnguard is the side suited to players on the good side of the spectrum and the Volkihar clan is more evil. If that doesn't matter and you simply want to know which side will make you more powerful, there's several considerations.

The Dawnguard will first and foremost give you access to crossbows and a series of side quests to augment them with powerful Dwemer technology eventually including explosive bolts. Crossbow bolts can also be forged in their fort. They also have specialized restoration spells that do direct damage to undead enemies, three unique artifacts (a hammer, a shield, and a war axe) enchanted with various effects useful against undead, a unique set of armor, huskies and armored trolls as animal companions, and several new potential followers. They would be the better choice for pretty much any kind of warrior.

The main draw of the Volkihar side is the Vampire Lord form. It grants several abilities that can be quite powerful, most of them magic based. There's also a couple of rings you can retrieve that boost your magicka and health respectively, and a third artifact that gives more power to your vampiric drain. Finally, they have Death Hounds that can be your animal follower. This is definitely the better path to powering up a mage character.

Finally, Serana (the main NPC) will be available as a follower no matter which side you choose. I hope this helps.

Thx for the reply and thx to you I'm going with Dawnguard