Honestly, if the extra $500 is not an issue, then pull the trigger on the Ultra. It will help you to sleep better.......no buyers remorse
Even with the SVS upgrade policy, you will need to pay shipping in order to return the PC-12 Plus (if you decide to buy that sub first). So if you factor that extra shipping cost, your difference on buying the Ultra now versus the Plus will be actually something like $360 (assuming a shipping cost of $140).
My two cents!
That reminds me, I used to have a rule: Figure out what you need right now, and then buy the next model up. it worked for a while until I realized that what I REALLY wanted was the model two steps up!