I am also one in the market for one such 15" allround notebook. I was intending to switch to Windows again because I dislike die Office situation in OSX and I dislike rebooting for games. Other options then the XPS 15 would be a new Samsung Ativ Book 8 or new Zenbook UX51VZ but they do take their time with updating those. I suspect the Samsung will be the best value.
XPS 15 as sold here only comes with the big battery if you buy the 512GB SSD version so it costs just as much as the entry level 2Ghz Iris Pro, or 600 less than the similarly specced 750M version.
Dell and its XPS line had cooling issues in the last version and over the years noisy cooling and all sorts of issues. This one is a new form factor and slimmer so I really need to see a test. For the same reason I don't like the Zenbook with its less than silent idle cooling. Samsung is great at cooling but they need to up their game on the SSD side (not that they couldn't given that they are the #1 SSD company).
Battery life claims don't mean much. We really have to see but I wouldn't be surprised if they beat Apple. They use a more efficient quad core and Samsung beat Apple with the Ativ Book 8 too (admittedly with a different display).
Aside from what we don't know for me there are some more differences.
Pros XPS or any Windows:
Cheaper
750M or better and no graphic switching issues, silent Intel GPU works in multi monitor mode.
Office is just better
Games work in Windows, no rebooting, no OSX gaming
Pros for MBP:
Resale value (even my current 2010 MBP seems to still sell for enough money to make up the price difference)
BetterTouchTool (I just like my custom gestures so much and don't know of anything similar in Windows, I don't know how I live without them)
It is still 16:10 doesn't have a touch screen. (I think touch screens are great on convertibles like the Sony Multi Flip, they are useless on a standard notebook in normal keyboard mode and require a glass layer to protect the screen from all the touching. I don't like glass. I want matte but MBP glossy is still better than the glass layer of the old MBP or the XPS 15. I think Intel's notion of requiring a touch screen is stupid and should be ignored. My hope is Samsung will stick with matte.)
Cons MBP:
I do play games and would want a 750M but I despise how just for the lightest of loads I get a noisier than necessary notebook in multi monitor mode. And no optimus in Windows. Iris Pro is I think great but not for my kind of use as I ever only need more than a HD4600 for games and literally nothing else (coding has very little GPU requirements

).
Warranty if included makes the price difference even bigger.
Personally I suspect that a new Samsung will be more of a contender for me than the Dell. A test of the XPS 15 might surprise though, it seems a fairly different notebook from the past ones. I really don't like the touch screen though. I think Windows 8.1 is great it doesn't need a touch screen. Works fine with mouse and touchpad. If it was a flippable hybrid, things would be different.
The high DPI of these screens are great. You can set any resolution in games and it will look sharp enough. I also think the DPI problems in Windows desktop are exaggerated, for most stuff it works just fine.
I am weighing BetterTouchTool vs GPU switching, quick gaming experience mostly.