Since you have the coilovers already your choices on springs is almost unlimited, this should be a relatively cheap upgrade. Talk to someone at BimmerWorld or KW about spring rates for your usage. Going back to your original question about antiroll bars the hot setup for E9x M3's and the 1M was actually to keep the stock rear and go with the larger front bar from the E93 M3 or the aftermarket. Kind of counter intuitive but it had to do with way the McPherson struts react to roll stiffness.
I went with the MCS 1-ways since my car is a daily driver, and they have been excellent. Same or better ride quality as the stock M units at low settings and they have the capability of being upgraded to 2-way.
What springs are you running with the MCS?