So much of “which OLED is better” boils down to settings and personal preferences.
For me, I own a Galaxy S8+, a Note 8 and an iPhone X. Don’t have my Note 8 with me today (it’s so big I find myself not carrying it as much) but I do think it’s got a terrific screen for most tasks.
But I do find my iPhone X to be the brightest. Here’s a side by side of the S8+ and the iPhone X, both max brightness. I find the text to be sharper on the S8+ overall, but the brightness of the X seems to make things pop better.
TrueTone is off on the X by the way. So is Bluelight on the S8+. Taken on my desktop with ordinary office fluorescent lighting above. Taken with an iPad Pro (2017).