Big difference is OLED has no backlight while QLED has an LED backlight. Since OLED has no backlight, they can be crazy thin.
QLED is brighter, though.
OLED has better contrast and black levels.
Always pros and cons. When looking at buying TVs in the past, I would get into reading comparisons online and it gets crazy overwhelming. Then, when I go the store, they all look fantastic.
Reality is, pictures are so crazy good now on all types of tvs, for the average Joe it isn't gonna make much difference. The technology is advancing so fast, they are making improvements that are imperceptible.
Frankly, they're all pretty close in the various metrics. Just b/c one is brighter, doesn't mean the other one is going to be dim. I'm sure if you looked at any good OLED or QLED in the store you would not come away thinking "wow, the OLED really does have better contrast!"
OLED supposedly has a wider viewing angle, but how wide do you need the angle to be? I'm using a 6 year old Samsung LCD and the viewing is as wide I can imagine I'd ever need to watch tv from. A lot of the specs are not practically relevant.