It depends on what you mean by build quality. The Dell - and I have one - is constructed to a more 'industrial' standard than the Apple, but variances in panel fit are less critical than on the Apple. Let's just say it's a less 'precious' machine than the MBP, although equally effective. Undoubtedly the Apple takes the style crown, but a laptop isn't just about how good it looks - at least, for me.
Assuming we're comparing between the 17-inch and the XPS 16, then the 9-cell *replaceable* battery gets me just under 4 hours of runtime in the same usage pattern that gets me just under 6 hours of runtime on the Power Saving profile of the 17-inch.
If we're comparing with the 15-inch... well, the XPS is a different class of machine. I guess in that case it becomes about what you need.
The only thing really I don't like about the XPS really is the screen, and I can say the same thing about all of the Aluminium Macs bar the antiglare 17-inch. The screen is just ludicrously reflective, and like the Macs, is unusable in certain lighting. Other potential dislikes include how long the leather accent actually stays looking good, as opposed to worn and tattered. Probably not a factor in my case as I don't intend to keep it longer than a year - 18 months on the outside, but definitely a factor if your replacement schedule is longer.
Costwise, and also in terms of utility - the XPS beats the 17-inch into submission with a GPU that's roughly equal, runtime that's not too shabby in real-life comparative situations, good general performance, superior utility in many respects and a ~thousand-dollar cost difference even factoring in three-year warranty and accidental damage cover. The 17-inch definitely has the style stakes tied up, but at the cost of lower durability in real-life use and a nonreplaceable battery that might result in usability issues, especially given that the runtime isn't long enough to be a true all-day companion for some (read: me).
Build quality variations? Out of all Dell lineups, I've easily had the most issues with the XPS line of notebooks - although not to a functional level that I've experienced with Apple - and it can be argued that in the case of Apple, it is easier to get purely cosmetic issues acknowledged, especially as you can walk into a store. I've had an isolated number of cases getting purely cosmetic, non-functional issues acknowledged by Dell when it occured. My single completely negative Dell call stems from a cosmetic issue I had with an M1330 where the supervisor was telling me that it was OK because his had the same thing. As you might imagine I was rather incensed and later I extracted an apology from senior Dell staff as well as compensation. You might not be accorded the same attention as I spend - well, quite a lot with Dell. So YMMV.
As for the comparison with the 15-inch, I personally wouldn't as they fall into different categories of machines.
I doubt if you're posting on a Mac forum that the decision will swing towards anything other than a Mac, but personally, as a sole machine I'd be depending fully on, I'd rather have an XPS 16 fully warrantied up than a 17-inch Macbook Pro with Applecare.
