'better' is relative. I own both, I can tell that the slim colours are more original, and exactly like it was displayed on PSP (same LCD screen). OLED looks more vibrant, but because of this it gets hard to read some text on the XMB when there are bright backgrounds. (Red, Orange especially).So PSP games look much better on the 1k OLED screen? Guess that's the best choice then.
It would be best if you could use both in real life and compare. Also, the slim build is flimsy and I've had analog stick problems with both Slim's I've used after just a couple of months. The OLED feels better build, but the front seems bit delicate because it's all glass. Use a screen guard or tempered glass.