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.
“Anna, my strong, terrifying Anna.” -Cas
"You lack the requisite spine and testicular fortitude to study under me" -Elodin