i like the idea of subtracting from either the head or the tail of a virtual list too; that's clever :)
With the xor solution, on the other hand, you don't really have that option -- if the input does not exactly match the input condition, then it WILL return whatever - with no indication that anything is amiss. Even slightly wrong input will give unpredictable, completely wrong output with no indication that anything is wrong.
What's worse is that it's trivial to game: as an exercise try making a malicious generator that takes an n of at least 4 and Target number, and returns (generates) an array (of size n-1) for feeding the XOR solution that shall get the xor solution to output the Target as the missing number. Further, the malicious generator, instead of generating an array with one missing number, actually shall generate a list which is as close to correct as possible, except that it shall (as an extra 'fuck you') in all cases contain the "Target" value -- twice.
btw thanks for the feedback on the sum thing :)