Most mining pools set that for you, BTW. And there has been some (unverifiable) claims that the main mining manufacturer, Bitmain, is pressuring their customers on this.
> Any claim that miners will fail to do what they continue to explicitly say they will do should be met with skepticism.
On the contrary, it's clear we're going to get a split, and in the past we've seen miners mine pretty much exactly according to exchange range on each side of the fork.
Annoyingly, lite clients won't be able to choose, because the BTC1 dev refuses to use the hardfork bit (the sign bit of nVersion in the block header) to flag their fork. Lite clients, unlike full nodes, follow the most-work chain without checking; since they anticipate hashrate majority, they seem to be trying to leverage that into a user majority.
It's going to get messy :(
That's unlike a full node, which fully validates every block and transaction on the bitcoin network on his own. Full nodes will insist on following the network rules they're familiar with, regardless of what the miners say.
The best available evidence is mined blocks, as they are unforgeable. Despite being under no obligation to do so, >90% of miners are currently explicitly endorsing the 2x agreement in their blocks[1]. Any claim that miners will fail to do what they continue to explicitly say they will do should be met with skepticism.
[1] See "Segwit2x (intention)" here: https://coin.dance/blocks