First of all the thickness of the rad will depend on your motherboard heatsinks. To be on the safe side, a 30mm thick will work with a set of 25mm fans, but you could get a thicker rad in if your motherboard layout allows that. I would take a 240mm rad because you can use the offset mounts in the top its also cheaper and will allow for easier tube routing because its more narrow than the 280mm one. If you choosing a 30mm one, take the alphacool st30.
The heatsinks on the formula are all over in the top they are also big, I would get a 30mm rad, to be on the save side. I think i have a picture of a 30mm one in my case with fans and you will see how tight it is.