I have have only used the premium flavors. For my upcoming order I am only going to try the free flavors with Splenda (Sucralose). The premium flavors are tasty, but I was thinking I chug my shakes so I don't need to have a premium flavor. I can tolerate pretty much anything though. I used to blend cans of tuna back in the day.
As for which sweetener, its really a personal preference. You would probably do ok with aspartame since you are already used to it. I personally prefer sucralose. I am going to request a few samples of unsweetened / unflavored powder in my next order to see if I can handle it. Would love to get off sweeteners altogether if I can handle it.