I have yet to play Dawnguard, I got it on a Steam sale awhile back but never started it.

The outcome of either playthrough is the same?

Goodguy Dawguard member = kill the evil vampire lord.

Badguy vampire = kill the vampire lord.

Is this correct?

Yes the narrative is generally the same, as Harkon's intentions are not ideal for anybody.

They're not vampire-oriented intentions?

They are vampire-oriented but the main focus of the DLC is helping his daughter Seranna, and saving Skyrim and Harkons intentions are not good for either so you stop him from his plans, would explain a bit more but I'm really tired .

Harkon has vampire oriented schemes, thing about it is that they sound really good when he talks about them (well, good for vampires) but when you apply reality to it then its a terrible idea.

The story is you helping his daughter uncover the truth about things and eventually learn how bad things would be if they came true.

No, because blotting out the sun would cause everybody to go kill the vampires. It's not in their best interests to cause a stir like that.

Harkon is one of those nice, reasonable guys who's completely whacko underneath. We all want him gone.

This. I was totally cool with blotting out the sun, I was also perfectly happy *not* being the big boss and letting him do his thing while joyfully playing the right hand man.

TvTropes basically calls that position "The Dragon" which I felt was fitting. But nooooo, he had to be a paranoid egomaniac and decide I was too dangerous to keep alive. I could have been the Vader to his Palpatine but the guy just had to go and be a big ol' crazy jerkface about things

I just kind of presumed they would've built the DLC with a well-thought-out angle to be played by vampire hunters, which would destroy the vampires... and an angle to be played by vampires, which destroyed the dawnguard and allowed you to terrorize the land. I suppose that would have implications for how the rest of the game plays out... I dunno, it just seems like it wasn't a terribly well thought out story to be played by two opposing angles.

Indeed, block out the sun, kill people, drink their blood, live in a vampire wonderland... I don't get what's wrong with that?

There are story options in the narrative where YOU can state your different intentions at times such as if you "want to help serana" or "stop harkon" or admit selfishness and say "forget you and serana the bow is mine". That last one was the one I took, it was generally how I felt at the time.

You can still do all of those things...?

Agreed to the bone. Was really looking forward to helping Harkon.

Thing with that was basically

1. People would rise up in response to the vampires.

2. The vampires would quickly run out of food on their made killing spree.

3. They'd still run out of food because producers of the planet would/could die of lack of sunlight to grow thus starving humans.