The con-20 is ugly, crude, and you can't see the ink remaining, but it holds more ink than the Con-50, and works reasonably well. The Con-50 is cooler, looks good in a clear pen, you can see the ink level...but doesn't hold a lot of ink. The good news is that if you are going to use the fine nib, you're not going to be going through ink fast.
Being able to see how much ink is in the converter is quite important to me. I guess the CON-20 would make me frustrated..