I feel like 4x is definitely the best method for PAL games by using the V Active 270 method, because that way you get 1080p which scales perfectly even on a 4k set. For NTSC though, I definitely think 5x looks much sharper on my set than 3x optimised. I understand that 720p x 3 = 2160p, but the pixels still have softer edges than they do in 5x. The only thing is that in 5x I have to use 1600×1200 and force my set to 4:3, otherwise the aspect ratio will be wrong or the top and bottom of the screen will be cropped.
Also, the borders don’t matter on OLED because they’re completely black like the TV’s frame. 😛