Caveat emptor - I am not a gear guy
At the beginner level, it shouldn't make much difference. The real questions are:
-What kind of snow do you get?
-What kind of terrain do you have?
-What kind of demographic do you have renting gear?
To the extent that you have more natural snow vs man made, go with the rocker.
To the extent that you have steeper beginner terrain, go with the rocker.
To the extent that you have higher percentage of renters taking lessons, go with whatever board the ride school wants to teach to. (if you go [off your] rocker, you'll probably want your instructors riding rockered boards too, so consider making discount boards available for them as part of the deal).
To the extent that your renters are going to be park rats, go rocker.
To the extent that advertising has an impact on your demographic, go rocker if only to advertise that you have the "latest" tech.
The key differences you want for first timers renting gear are a board with a soft flex and high bevels. At our resort , we have a mix of Burton LTR boards (for first timers) and more normal flex/bevel boards for intermediates. The knock on rockers is that they make turn entries less precise. The plus side of that is that they are easier to start turns. As you've noticed, other than that, there is not a huge difference until you get into powder snow.