Not sure on sound as I have not really heard enough of both. But I believe the suprelux's are removable caps and there are Omni and hypers for them (although I guess they are short in supply) which give you a few more options. The C4's are fixed card caps If I remember correctly. I've been thinking of the superlux's as a inexpensive rig myself.
Both mics allow interchangeable caps. both mics first came out with only card and omni with the availability of a hyper cap set for the future (the hyper cap is the most difficult to make, and to make well I might add). I am almost certain that they hyper caps are now available for the luxes but the c4 hyper caps are delayed for the time being. However when you buythe c4's you get the card AND omni caps whereas my luxes only came with card caps.
Coincidentally enough I have both the luxes and the c4's but I just got the c4's and have not taped a concert of any type yet. Let me know if there is any interest in a listening comparison of my home stereo.