I'd go with Shields.
Shields has all the tools (minus striking) to not only give GSP a tough fight, but beat GSP.
Where as Randy only has 1 tool - wrestling, and with Machida's sumo and very solid clinch/TDD game, I'd say it is probably enough to defend from Randy's control,
Both can definitely do the upset, but Shields has a lot more going for him against GSP than Randy does against Machida.
"You lack the requisite spine and testicular fortitude to study under me"
"You do not know the first note of the music that moves me."