I had a pair of Burton Malas that had a regular strap and then updated to a set with the asym strap. You can definitely tell the difference. Better cushioning and support with lateral movements.
"Is it worth the upgrade?" I'd concur with Mrjimyjon. If you already have your '12's paid for then just hold on. If you like the new colorway AND want to try the asym strap AND the new high back then just see what you can sell your '12's for. If the difference in cash it would take to get your '13s wouldn't be too much then go for it!