If I were Cancellara and I absolutely wanted to give both the Tour and the monuments a go, I'd go for the Tour first. But it's complicated.
First of all, I'm afraid it might be a bit too late for him, although on the bright side next year he'll be just one year older than Wiggins when he transformed. Ideally he should have given it a try already, but it was impossible at Saxo. Next year things may change depending on where he goes (if it's BMC, I don't see him displacing Evans with no GC results to show). Frankly, I don't see it.
Now, as for the monuments, MSR, RVV and PR are quite obviously and empirically within his possibilities. LBL is a bit too hard and explosive for him, I think, but GL might be more suited to his characteristics if I'm remembering the parcous right even though I haven't been able to watch it for years so this is probably ridiculously wrong. At any rate, turns out he already has the monuments thing more or less covered, so I don't see it providing him any extra excitement.
Short stage races are fine but he's already won TdS and I don't see any of the rest improving significantly his palmares. Yeah, winning the Dauphine would be good but if you can win that you might as well go for the TdF, right? Romandie, Basque Country and the others just don't look like very exciting goals for someone like Cancellara.
So I'd say, TdF. I'm very skeptical about his chances of doing good in the GC, but it's the only higher goal I can see (WC aside).