If you are going with an optic, I'd definitely go with flip-ups. The fixed HK sights are a bit intrusive IMHO. Flip ups won't interfere with the front sight. For the rear sight, it depends on the position and, if it hangs over the rear sight, the height of the optic. I've got KAC flipups on mine and they don't interfere. My optic overhangs the rear sight, but the optic is high enough that it easily clears it.