Julie Chen and Jeff Probst just walked into your house and said, "We would like for you to be on our show, but you can only be on one." Would you go on Survivor or Big Brother? ...or ask them why the hell they thought they could just walk into your house...?
For me, I would probably go with Big Brother because I'm an extremely lazy person. Survivor would be a lot of hard work that I probably wouldn't be willing to put forth effort for.
On the other hand, I think Survivor would be more of an experience. I also believe I would enjoy the Survivor competitions more then those of BB.
I think it's a really tough decision. In the end, I think I would just make Julie and Jeff fight to the death, and would join the show of the winner.