Opinion -- both are equally clear at first glance.

I personally don't like the "Action unless Condition" form, it's too easy to eye-skip over the the second clause, partiiculary if the 'unless' is on the next line. I know that the complier/interpreter will fold the various constructs into an single "action if condition" form; but I tend to look at the condition as the more important part of the statement, so I prefer it first. Tastes Differ.