The super mutants we see in the demo do grow into behemoths, meaning that they could be of the east coast strain, however they have dark green skin like the west coast strain. They also seem to be a little bit more lanky and creepy, however still intimidating.

So are they west coast? DC strain with stylistic changes? Or a new breed of mutant? What are your thoughts?

Im inclined to say style change, we seen other cases of this and its only natural consider the gap between games and the graphical advancements.

It's probably just the Vault 87 mutants made to look more like the Mariposa mutants. There's a lot of stylistic changes in this game to make classic Fallout icons that appeared in 3 closer to what they were in 1 and 2. The vault suits, the power armor, the vertibirds, etc all resemble their FO1 and 2 artwork, as opposed to the FO3 artwork.

They just retconned the skin colour of the east coast SM's. Bad design decision IMO as the difference in skin colour helped differentiate the two strains of SM's as easily as by just glancing at them.

Same mutants; different style. Kinda like how elven armor in TES is visually different in every single province the game has been set in...

This.

I suspect after the backlash they got for making so many new things, and making so many old things look different, they are trying to bring Fo4 more inline with the past games to sort of ease that out a bit.