I use AMSOil in my Rotax 2-stroke engines, but...
...in the Mercury 240EFI I continue to use Mercury/Quicksilver Premium Plus. As you note, there's really no cost savings so that's not a motivation. I also talked to someone at AMSOil and while they aggressively recommend their products for almost every engine you can think of, they are "cautious" and "cagey" when it comes to the 2.5L and 3.0L Mercury engines. That really made me think.
I have tried other marine 2-stroke oils and they smoked and stank much more than PP. Finally, while I know manufacturers want to sell you their inflated and expensive products, Mercury knows their engines better than anyone and PP is what they recommend. Not a deal-maker but definitely a consideration.
Since there is no financial incentive to go elsewhere, and there are definite smoke and smell incentives to stick with PP, and there just might possibly be some additive advantages to the Mercury product, I've chosen to stick with PP.