As you can see from my signature I own at least one of each major type of scope. Every type is a compromise at some types of astronomy and the best at others. Newtonians are the best bang for the buck while APOs are the best bang for the size. SCTs are good at everything but not best at anything. Like others have said the difference between achromats and true APOs (not always scopes described as APOs) means that a 10% increase in performance requires a 1000% increase in price. Mass-produced verses craft builders. Ferraris verses Chevys. While a Corvette is the best bang for the buck, one ride in a Ferrari and you will know why it costs so much more.
If you want to see the best a Newtonian can do you need a craft built 20" and larger one which coincidentally is in the same price range as the 6" and up APOs. And don't even think about the mount issues. Same price ratio holds true when comparing AP mounts verses mass-produced mounts. On the other side of the equation not everyone needs, wants, or can afford the best of everything. That is why the market covers every price range. The mass-producers don't make high end items because they can't make them that well while the craft makers don't make low end ones because they couldn't make them fast enough. High end equipment holds its value over time while mass-market things don't.
If you want to know the difference you need to look thru some of the high end scopes. But I caution you that without realistic expectations you will suddenly find your equipment likely suffers in comparison. Might cause you to suddenly want something better. Luckily these days there are scopes in the middle ground that can give you a smooth progression towards the high end without having to take a big leap all at once.
My philosophy in astro gear used to be "no type of telescope smaller than the last one". But by the time I got to 16" Newtonians, 11" SCT/Maks, and 6" APOs/EDs I got old enough that I could not go larger due to the physical requirements to setup and use them (70yo). While I still have a lot of that gear I am downsizing to what I know I can handle physically. Still buying and selling gear to figure out my limits.