I followed a similar path as you Xenobiotic but I've changed my mind a hundred times now I can't nail down what I want to do. I want something I can do easily, my first choice was the fists because their my favourite legion but they ain't easy to paint, my most recent choice was the raven guard, but they rely on highlighting and my highlighting skills are rubbish.
My marine chapter is the warmongers because I find painting green and blue the easiest and I can also do grey to but it may be a bit boring.
I narrowed it down to the following.
Ultramarines
Night Lords
Salamanders
Alpha Legion
Word Bearers
Space Wolves
I've ruled out the Emperor's Children because I don't like them at all. The others I'm not so sure about.
The Night Lords are cool, but I've never been drawn to them and they seem pretty popular which is a turn off to.
Salamanders, there's not much to go on with these, and I really don't like the coal black skin and burning red eyes.
The Ultramarines are a difficult one, they used to be the poster boys but are they still, are they the new uniqueness or are they played widely? I still get the feeling they are, but I know I could paint them easiest and their activities in the heresy are getting more interesting.
Word Bearers, definitely not popular which is a plus as not many will choose them. I don't especially like Erebus and Kor Phaeron, I actually liked Argel Tal though, but their combat prowess seems a little crappy, every time I've read about them they are pretty fodder like. They also turn red, which I don't want to do.
Alpha Legion are interesting, but I struggle seeing them as a combat legion they are just so secretive and shadowy that would they ever really translate that onto the table top? I like proper armies with distinct tactics, the Alpha Legion come across as winning things without getting dirty.
Space Wolves are cool but they are overly popular, and would require a lot of bits to get the character into them, something I don't have.
So you see, it's difficult for me to decide what to go with, I don't want mess the army up, I want it to look nice and I want something I would enjoy playing and imaging it actually happening in the Heresy.