I've got both, and have run the matrix in 2 matches so far. I'll be doing a comparison YouTube video here shortly. But the long and short of it is, the ACC is more adaptable. I can get it both lighter and heavier than the matrix.
I really like what the matrix did with the trigger finger support. I thought it would be a gimmick but it actually works to put your finger in the same place every time.
The new forened with the spigot is about as long as the ACC which is nice.
It comes with MPA new adjustable mag latch but I'm not sure how well it'll work as it's just one small set screw.
On my ACC I have the folder which I really like so when I fly to matches I can use a smaller pelican or pull the bolt to clean, it's easier. I also prefer the magwell design on the ACC because it's bigger and works excellent as a barricade stop.
I don't think you can go wrong with either, but if you're looking for something super versitile (especially since MPA hasn't released the matrix weight kit yet) I'd go ACC. If you love the look of the Matrix like I do it certainly is an awesome chassis as well.