I asked this same question a year ago and ended up getting the f4 IS. I have some regrets and still think about selling to get the mk2. But that is just my greed talking, the lens itself is great. It really boils down to, do you need the 2.8. I wish I did, but even the f4 is too bulky for me to carry around, so even if I did upgrade to the 2.8, I can't see myself using it more than my f4.
OP, it seems like the 2.8 would be a better choice for you, if you are planning to shoot indoors. The f4's IS is great, but it might be too slow indoors. Correction, it WILL be too slow for indoors.