It's a very tough call Bears fan. Clayton will be more solid week to week. Thomas could be big and small and he's been dinged up too. Your WR corp is solid as far as starters go. You might want to look at your WR bye weeks and see which has matchups that make you feel more comfortable on the weeks when you will actually need to plug them in.
I'm in the slight nod towards Clayton category, the Rams will always be throwing a lot all season long. Sorry, almost more questions than answers.