I am not a Rollei aficionado by any means, but I was in your shoes 4 years ago and decided to get both, the 2.8F and 3.5F and both are Xenotars. There are no image quality difference vs Planars, but folks who repair them suggested that Xenotars are less prone to separation.
Personally I agree with those who say that condition is more important than the model, with one caveat: the screen needs to be user-replaceable. Stock screens aren't bad, BTW. In fact, under bright light I prefer them, but they are on the dim side, that's why my 2.8F has a screen from the 2.8GX