For me none at all.
The only differences between the c and f versions are
- sign of the power-up timer enable bit
- amount of RAM
- code protection (and ease of defeating the code protection).
So get a 16f84, unless you can get a 16c84 much cheaper.

Wouter.

----------
> From: Gabriel Aurilio de Oliveira <gabriel@INEP.UFSC.BR>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Flash Memory
> Date: Tuesday, August 04, 1998 22:25
> 
>     Hi,
> 
>     I'm a beginner in the Pic microcontroller world and I have a little
> question about Flash Memory:
>     What's the difference between the Flash program memory of the
> PIC16F84 and the EEPRON program memory of PIC16C84?
> 
>     Thank's for help.
> 
>     Gabriel AurŽlio de Oliveira