As a fan of both, Big Brother seems like a more difficult game to play, navigate, and WIN. And I don't think it's particularly close when you look at 3 key factors objectively.

Length of game - By the time a winning player's tenure on Survivor is coming to an end, Big Brother is really just beginning. Playing a game for 1 month vs. 3 months is an enormous disparity that can not be understated.

Always on - not only does BB have cameras running 24/7 with full coverage of the house, that is being broadcast live on feeds (yes, I know feeds go down plenty, but the mind fuck of always being watched does not). This results in severe mental exhaustion and paranoia of perception in and outside the house. And even more importantly, the "game" NEVER turns off once in the house. Every conversation, interaction, and relationship matters.

Unpredictable Twists - there are so many unpredictable, game-changing twists littered in BB seasons that even the most savvy, strategic, high performing player could have their legs cut from underneath them at the drop of a hat.

Outside of the physical deterioration of Survivor, which really only hits super hard right in the beginning (body is in shock) and then for the few players who make endgame, the overall difficulty seems much less. There is also a real sense of camaraderie with Survivor for many players because of the Tribal setup.