Hi Luke,
I general the grey boot was for the mark1 C5 - the first main batch, This was later on changed to a white boot with some minor mods that was done to the C5 i.e. wiring changes, gear box material (brown to cream), etc. The basics are all the same and quite a lot of boots over the last 30 years have been interchanged.
Nowadays it is down to personal preferance and what you get when you buy your C5.
Hope this helps.
Lance