Which significant differencies between KL an C lenses for Mamya RB?

Hi,

I live in Italy and I've recently bought a Mamiya ProS camera with a 90mm
sekor C lens. I'm currently looking for a 50mm C lens. For the future I plan
to expand my system with more lenses upon my needs (I need at least a
telephoto). Are there any significant difference in terms of quality and
resolution between KL and sekor C lenses? I like generally low contast
pictures but always higly detailed. Since I'm a student I'm on a budget,
but if it is really worth the money I could consider to wait until I can
afford KL lenses. Let me know your opinions.

thank you