Thank you so much for both of your opinions !

If the difference between the Contax and the Nikon isn't major... then I think I'd rather stick with the F3 which I'm used to and really do love working with... I will check out the Zeiss Ikon as well, thank you for suggesting that.
I have also heard very good things about the Mamiya 6 and 7 so thank you for suggesting these, but I really like both formats, so I like to have one camera of each.

I am currently using a 35-70 mm AF Nikkor lens, I like its versatility and it always allows me to take the shot I want, but sometimes when scanning the film I have the feeling the global sharpness could be better... but maybe it's just me, it's hard to know when we're reaching the limits of the scan or the limits of the lens...

Won't buy on Ebay no more no... I plan on getting the new one at B&H, I know they're serious, got my Mamiya, in Used condition at their store and never had one problem with it in 5 years of using it.