This is a very wide question and comes under the "Loudness war" that's been going on for 20 odd years since technology has allowed for louder and louder music, always pushing the average volume of music towards -0db.
Personally, more headroom is always better, it allows the quiet parts to be quiet and the loud parts to be loud. -6db may seem to have more "oomph" to you, but try reducing both tracks to the same loudness and then listen and I'm pretty sure you'll prefer the -9. This is psychoacoustics, not necessarily my area of expertise but I've read about lots of aspects and I advise you do the same.
When music is louder, it is perceived to be "better". That's just how things are. But the mix will be of a better quality if it is quieter and THEN you just turn your volume knob up.
Of course, your genre of music uses peaks heavily in the beat, so having a flat waveform doesn't actually benefit it at all. As much as many people say you have to mix electro to louder levels, I disagree. All music benefits from more headroom.