Overall, the physical rewards (weapons, etc) are better if you choose Dawnguard. However, you cannot regularly use the powerful Vampire Lord form until after you've completed the main questline, or else the Dawnguard members will not accept you.
Yes, once you choose the Dawnguard, you can become a Vampire Lord in order to enter the Soul Cairn, although you don't have to.
In my opinion Dawnguard is the best choice. First spoiler warning, you can acquire the vampire lord transformation in both sides. After you destroy the vampires with the dawnguard you can ask Serena to bite you if you don't get her cured. Dawnguard is also the only legit way to get the crossbows and magic bolts.
the main benefits are the vampire lord skill tree, and accompanying powers, as well as a general boost to magica. Without mods, it's not worth it imo. The only real benefit is the Necromage perk that makes spells/enchantments 25% stronger on undead. Since you are now undead it effects you.
At lower levels, Werewolf is maybe a better choice, because it's easier to level up, and Vampire Lord won't have as many advantages. However overall, Vampire Lord is a complete power trip. At level 46+ you get an extra 250 health, 200 magicka and 100 stamina.
Lean meatsRemember Myplate.gov. Try to keep each meal balanced. Look for lean cuts. In general, chicken, turkey and fish are going to be your leanest meats. When choosing beef, make it lean. Substitute out the fattier cuts of beef, and choose round, chuck, sirloin and tenderloin (also the leanest when it comes to pork).
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