Few things to consider...
I agree with OP; matching carbon fiber in the front and back will look best. I actually think side skirt extensions are a must in this case too, otherwise you only see the one piece of carbon in the front or back from any angle. However, if your car is low or if you're the type of driver that tends to scrape the front lip for whatever reason, than carbon fiber can be a NIGHTMARE. It looks really nasty when it becomes damaged and it's much more expensive to replace.
Alternatively I like the dark grey satin plastic look if you happen to have the same dark grey plastic window sills. It also matches the underside of the mirrors and other details in the front bumper, etc. I have a really low set up so I went for the satin plastic lip and diffuser, and I don't cry when I hear it run. I also and have wheels with a satin finish so for me the plastic looks cohesive. You can also paint the plastic one gloss black and that looks really nice, especially on a black car, or if you have the gloss black window sills.
You can peep my build thread for more info. There is a pic of the msport lip in the last post.
http://www.bimmerfest.com/forums/showthread.php?t=915596&page=8
Good luck picking -- look forward to seein pics!