First of all, you can gain more vampire perks and powers in the Vampire Lord skill tree. To access this, use your vampire lord power to turn into a Vampire Lord, then press the button you'd use to open the screen with Magic, Items, Map and Levels on it. I use my Xbox remote so I don't know the PC binding. You'll need to kill people with the magic in your right hand to advance this skill tree's individual XP system. When you've got a perk point, just spend it in the same way you would normally with skill points.
As for the second... I think you become more powerful in the Magicka side of things... especially when you find the blood rings and when you use the Chalice. But at the same time it makes it harder to battle some dragons due to the weakness to fire. Also, the whole weakness to sunlight thing makes things harder.
All in all, the Dawnguard doesn't really leave you with any long-term perks, aside form a crossbow and some fancy armour.... but choosing the vampire storyline leaves you with some pretty powerful things. You can't get any followers from the vampire side of things, aside from Serana, because as I said, the Vampire side of things is a much more individual style, whereas Dawnguard is more team-orientated.
Hope that's useful to some degree.