Impressive site Pinkness.
Though I would think black background would suite your site better as pink is rather too soft for the content, but then you would lose the title's theme.
I am sorry if I also have not much experiment with the Bigmas you are interest on, but wide angle being one of my personal loved fields of photography, may be I can share some thoughts with you, in respond to your request;
The main problem I see with your need is, or rather are 2; First the full frame 5D, then your need for high quality. Well I sincerely hope your wallet is fat enough
1) Your best budgetwise bet would be L 17-40mm F:4 or even better if money no obstacle, L 16-35mm F:2.8 that gives you additional F stop. I stay with Canon's product as I have tried them, very happily indeed and don't know much about wide Bigma though have heard a lot good about them especially the 10-20mm.
2) Sigma circular 8mm fisheye F 3.5 Check this
LINK if not seen the specs before.
3) Here comes what I really want to suggest to you; Enjoy your 5D and save its large sensor form dust by less lens change so instead invest on a second body. Get a XTi or if too rich, 30D that opens the door to variety of EFs lenses, then adding an extra body adds that security, should things go wrong in an important photo session like battery, memory and less likely primary camera's failure, you always have the plan "B". If you go this far, my next suggestion is Canon's 10-22mm EFs that gives you the wanted quality at an unbelievable price. This is one keeper lens with warrantied fun.
3a) Now that you have the better versatile mount of XTi, you may want to try full 180 degree fisheye at a much cheaper price; So Peleng 8mm 3.5
LINK
LINK-2
A real real fun lens specially for kind of picture you have in mind.
Few things to remember, It won't fit your 5D (Mirror hits the back of lens so non operational)(Ah those days when mirror lock up and accessory visor on flash shoe was an option, talking of Nikon's Early Fisheye
LINK)
Pros: Cheap. Very sharp. Comes from same manufacturer that used to make the Ex-other-superpower's military stuff.
Cons: Manual focus (I consider it a plus under low light situations, no un-necessary hunting for focus).
Then poor control of internal reflections due to poor quality of lens coating. So you may not like it when spot lights are pointed toward the lens, and let me tell you it is very difficult to get things you don't want, out of field of this lens as it seems to capture everything around it.
4) (In same like of an additional camera body);
If budget is a limitation, you may want to consider a good Add-on-lens to get your fisheye, like Nikon's FC-E8 that goes on several coolpix point and shots and gives you a true circular fisheye view, with full AF power and also TTL flash, and all electronics a modern camera is expected to provide you. The combined cost of lens and camera should be well under $500, Cheaper than any DSLR lens alone.
Quality not as good as DSLR but still very acceptable especially for web.
LINK
5) Buy one of Old Nikon gems
LINK Or
LINK-2 then get a Nikon to EOS adapter with AF chips that gives you "Focus confirmation" light (To make it more electronically fancy, not a big plus otherwise as you still should focus manually)
LINK
You can do the same thing with Olympus 8mm lens too but I consider Nikon's glass a lot better than oly and almost similar to Canon and not any inferior. Also both give you F 2.8 that would be very handy in low light situations that you have in mind.
LINK to Olympus.
Hope this was useful to you and gives you some new ideas and alternatives to achieve what you have in mind.