Boolean constant for assert in if..else block
Throwing an assert error is probably heavier than throwing an Exception (as Groovy will parse the assert inputs to give you a pretty output string)

And unless you are going to be running this hundreds of times per second, I wouldn't worry about it either way...

A way of using assert that gets round the warning (and provides a nice error message) might be to do:

assert condition_1 || condition_2, 'Expected at least condition_1 or condition_2 to be true'