You’ve summed it up perfectly. T15 is a solid performer (with universal clamps btw).
The StringMaster is just that step ahead in pretty much all aspects. Better clamps, better bases (a lot!), better build quality overall and better mechanics to mount the racket. In my opinion, the extra money in comparison to the T15 is well invested.
Ask yourself the question how long it will take until the extra money for the Deluxe will be paid off? This is a one-time investment that will last you for many years to come.
Which Gamma machine do you own currently? Could be the cheapest option to stay with it and just add a WISE. I don’t expect the T15 base machine to be a serious upgrade from the Gamma tbh.