I have seen the usual contact structure as either the kernel of $dz + xdy$ or the kernel of $dz - ydx$. They are contactomorphic, of course, just rotated.
However, I was wondering if there are any particular reasons why one would use one form over the other as the ``standard''? I had googled it to no avail. I did find that Etnyre states that "many authors prefer to use" the latter, but with no reasons named.