Right off the bat, you could probably save yourself some money by NOT getting the Intel i7 processors. They're quad-core, they're powerful, but they're just not necessary for the stuff you say you want to do, unless you plan on doing some heavy 3D rendering and modeling for engineer work or something. An i5 will do the job just fine, saves you at least $100, generates less heat, and gives you better battery life.
Second, where are you getting your ENVY 14 battery life numbers from? Most reviews out there max it out at 4 hours on the main battery, with the additional slice battery giving you at most another 3 hours, for a max total of 7 hours. A 12-13 hour battery life is unheard of for computers with this kind of hardware. If you want to play games while away from a socket, expect your battery life to drop dramatically.
Both computers would be fine for playing CoD: Black Ops on medium. The graphics cards are sufficient.
The HP ENVY 14 has, in my opinion, better aesthetics; it tries to mimic the MacBook Pro's unibody case, and is completely coated in anodized aluminum. This means that it's slightly heavier than other notebooks its size, but at the same time, it's incredibly sturdy and has a very high build quality. It also has a backlit chiclet keyboard, if that interests you. The XPS 15 on the other hand seems to have some aluminum and some plastic in its case, and is still heavier than the ENVY. Also, I'm pretty sure it's keyboard is just the basic, old-school, non-MacBook-ish one and probably doesn't have backlighting.
The only key difference, besides the aesthetics of the two laptops, is screen size and quality. The HP ENVY's screen resolution is 1366x768, while the XPS 15 offers a full HD, 1920x1080 resolution. The XPS 15's hardware offering seems more powerful as well, but this is likely at the cost of battery life. Personally, I value portability and battery life, and I don't want a laptop that's overly powerful at the cost of those two--and I just like the way the HP ENVY 14 looks, so I'd probably go for that. (Just to keep it in perspective, I'm NOT an HP fanboy--I don't like any of their other products, and I'm typing this up on an old Dell XPS M1330.)
I hope this helps. My final suggestion is to look up some pro reviews on notebookcheck.net, pcmag.com, and other computer review sites before you make your final decision.