I have an Air and have just got rid of an X300, but this whole class of 13.3" machine is IMO in the half-ass area - especially the Air, but that's an argument for, well, many, many other threads.
X300 is not hugely short of a Sony SZ once you deck it out with everything, and the Envy 'starts from' 1.53Kg's. I'm sure the Envy will have significantly less build quality problems than the Air, but ultimately it's too heavy for what it is (a 2.5Ghz Penryn, dual X3100/8400m, 6-hour SZ's weight 'ends at' 1.79kg) and it's also kind of uninspiring-looking to me. Perhaps it's the colour but it just doesn't do it for me.
But it has some really nice touches - The power brick is one of the most innovative parts of the Envy. It incorporates a wifi access point with an RJ45 jack, so wherever you're charging it (with higher wattage than the chronically slow-charging Air) that has a cabled network port you can get online with the Envy. That is a great idea for roving people.
While the Envy doesn't tempt me, the Omen on the other hand - it looks great. I already own two 'Skulltrail' PC's at the moment but once it's available on these shores I may just have to shuffle them off for some other use and buy a couple of Omens with a similar spec.