I really want to be a vampire lord but I've heard that dawnguard is better which one should I choose?

I think it depends on when I play personally. If I become a vamp or werewolf earlier in the game, I think it's cool to cure myself later. But if it's late in the game, there's something different about that. Like it's less of a mistake and conscious embrace of the darkness.

Just my 2c.

I've only played vampire lord, and enjoyed it. But it's up to you.

One then the other for the replay value.

I personally prefer Dawnguard, but there is a way to join Dawnguard and still become a vampire lord. It's spoilery though, so can't say how in this section.

Depends from your character's morality. The is no room for a good or even gray Volkihar vampire.

My vampire was good. He only fed on bandits.

Well yeah, i mean most of their radiant quest are evil in nature and of course there is the whole Volkihar castle. That place is downright EVIL. For me personally it was impossible to justify my character's involvement with a group who keep enslaved humans inside cages and drinking their blood while they are alive.

Lol i'm trying to picture in my head a good Dragonborn having a normal civil conversation with Serana while three vampires next to him are drinking blood from a zombified human. Hilarious.

It's hard for me to play bad in this game too.

But my first foray into TES would have definitely done it. He was basically just a psychopath with no rhyme or reason (and Argonian).