The SL8 is overpriced, and ugly as can be (and I say this being an old H&K-lover from way back), while the Robinson M96 is overpriced and svelte, as well as modular.
My vote is for the M96, which uses any mags that will fit the AR15.
I've shot the M96, and it's easy to shoot and accurate. I hope to get one soon. Like you, I'd considered the SL8 at one point, but decided that I just wasn't sufficiently interested in a rifle with so many "compromise" features (funny light gray color, thumbhole stock, proprietary 10-round magazines, etc.), and decided to look elsewhere for a .223. Best of luck.