Rokinon or Opteka?

I’m thinking of getting a fisheye for my 550D this week or next week and I’m not sure which to get.

Rokinon 8mm or the Opteka 6.5mm

I’ve researched a bit and people say they’re exactly the same, just a different name. But what is the difference between the 8mm and 6.5mm? The image of the 8mm does look a bit more sharper than the 6.5mm but maybe that was the video.

What are your thoughts/opinions?
Also do you know any sites where I can buy these for a decent price?
Cheers!