The longest proof of work chain is the consensus mechanism.  

Not really. "the longest chain" is ambiguous. It should really be "the longest valid chain", and then you need to define which coin's concept of "validity" you're using. Every client follows the longest valid chain - that's precisely how they decide which chain to follow.

So assuming the question is "will my node follow the longest valid Bitcoin chain?", then the answer is "no" for XT once BIP101 is activated. At that point XT stops following the longest valid Bitcoin chain and starts following the longest valid XT chain, whereas the answer is "yes" for Bitcoin.