Eliminating lots of tech mumbo jumbo, "QLED" is really just a conventional LCD panel with different backlight technology which generally delivers improved color reproduction and many, many localized dimming sectors, making it much better at representing "truly black" parts of the picture than a non-QLED LCD. It cuts way down on graying and backlight bleed.
OLED really is an entirely different display technology, with a tiny LED making up each individual pixel. It will always be objectively superior at representing black better (the pixel is simply off, there's no light to be had) and have better contrast ratios. Whether that means they're worth the additional cost to you is subjective.
When I researched TVs earlier in the year, and looked at many side by side, there was no chance I could justify the cost of a true OLED panel especially when most of the TV we watch is stuff from the 90s and early 00s. The small amount of modern 4K content we watch looks great to me. I'm big into vintage audio and hi-fi, so I'm a nutty stickler there, but when it comes to the picture, the Samsung HDR QLED TV we got is more than impressive enough for me. I'm very happy with the quality of the HDR and how well it renders black.
I'm very satisfied with the panel in our Q70A, and while it's been many years since we had our Samsung plasma, I can't recall anything on that particular TV being as good as this one.