I use a mixture of C, CT*, CF and CB lenses.
You cannot tell the difference between lenses of the same focal length / design between the versions, truthfully.
The difference will almost totally be in how easily the lens flares. Use a hood and it shouldn't be a problem. (Supposedly) there are differences in color saturation between the generations, but I would be surprised if it is noticeable, even in side-by-side tests.
C and CT* are identical except for coating and color.
CF is identical to CT*, but in a different barrel and shutter design and with a unified B60 filter attachment.
CFi/CB are in yet a different barrel design with rubber grips, with different internal coating, and a different main shutter spring.
There are some other little variations (CF and CFi lenses are much better to use on an F series body), but mostly nothing to worry about.
Some lenses were only available in certain designs:
The 120mm 5.6 S-Planar (early variant, different from the later f4 Makro Planar) was only available as a C lens.
The 50mm FLE and 180mm f5 are only available as CF and later.
(and so on... info is easy to find on what is available in each design).
The main advantage to newer lenses is that they are less likely to need a CLA, and some prefer the handling.
The main advantage to the earlier lenses is that they are beautiful and smaller. They are slightly more costly to service, but cheaper to acquire.
Personally, I would prefer to have everything CFi because they are smoothest to operate and are less likely to have seen heavy professional use. I far prefer the 80mm lenses in C/CT* though - they are smallest and most elegant of all the designs.
Buy what you can afford and what appeals to you and shoot!
