I've driven both. The turn in over stock with both is incredible, it'll be the first thing you'll notice. On the street and most autocross courses the M030 will be as stiff as you can go without compromising handling. On a race track (which is much smoother) the Bilstiens will out perform the M030s hand over fist. The M030 kit is 30% stiffer than the stock suspension so even on 17s it will be stiff on the street to say the least. I think that the M030 setup is on the verge of over powering the chassis stiffness but I'm sure I'll get some disagreement here. The biggest draw back to the M030 kit which others have brought up is that its not rebuild-able. The biggest upside is that you can still qualify for "street class" in SCCA autocrosses, or at least that was my biggest upside. There's something to be said about beating someone with a "stock" car on the track, and the M030 offers just that.
All I can say is