I haven't cracked open the case on the E6 (yet) to see what's in there, but the pre-amps sound OK to me. I haven't given them a real good workout. They are quiet.
I haven't used the compact4, but I'm generally leery of those smaller mixers. I listened to a lot of them, the Yamaha, the Peavey, Behringer, Mackie, Alesis... I liked the features and sound of the SoundCraft best. Of course when I was looking, my price point was $150, so that limited me.
The SoundCraft has long throw faders, vs small knobs on the other units. I do think the compact4 has solo buttons, which is a huge plus in my book.