Hi Shaggles,
F4 is 'better' for photography, and therefore i'd recommend getting the f5 if you plan visual use. You will need to use a coma corrector for the f4, but you can get away not using one at f5 - well i don't mind especially visually.
I have a 12" GSO tube (f4) and a 12" Skywatcher goto dob (f5). I agree with Marc that you may want to atleast consider another brand unless you don't mind making modifications to it the scope to make it useable.
I
had to buy stiffer springs for the GSO and with the skywatcher i could get away without buying stiffer springs - in fact i haven't upgraded them and are happy as it is. collimation is easier in the SW too. I also plan on buying an upgraded spider vane for the GSO as it is quite soft and moves very easily.
of course if you are planning on putting it onto an eq mount in the future at 1500mm in length the f5 will be pretty hard to look through.
I had no issue on my f5 SW goto dob for astrophotography.
http://www.flickr.com/photos/80336656@N07/ although i did find a coma corrector was needed for deep space stuff.
some people like tinkering and upgrading parts so it might depend on what type of person you are.
You might want to consider a 12" f5 dob, then buy an 8" f4 newt for photography- they go pretty cheap.
All the best
Rusty