I've had the XPS 12 since yesterday and have been able to use it a few hours.
This vs the Surface Pro 3 is a bit of an apples vs oranges comparison since they are different classes of devices. SP3 being mostly tablet with a type cover and the XPS 12 being mostly laptop with a convertible display. And after using the SP3 I alread came to the conclusion I need a convertible more than a 2-in-1.
Displays...
I really do love the Surface Pro 3's 3:2 aspect ratio. The XPS 12 uses a typical 16:9 and has a slightly larger 12.5 inch screen.
The pros for the Surface is the 3:2 ratio and higher screen DPI makes it great as a tablet and comfortable to use in portrait mode.
The pros for the XPS 12 is the lower resolution means I can run without DPI scaling and don't have to deal with poor scaling issues with desktop apps. And the standard aspect ratio allows me to play desktop games at lower resolution and be able to use a lower resolution that matches the 16:9 aspect ratio. With the SP3 when you use a lower resolution you usually can only pick lower 16:9 or 4:3 aspect ratios, there are no lower matching 3:2 ratios to choose, so you have black bars on the screen.
The XPS 12 has a doozy of a con though. Many, maybe all, the displays suffer from an image retention problem. I saw this as a commonly reported issue on notebook forums before buying, but was hoping mine wouldn't have it. It does. Though at first I didn't realize I had the problem, because it really is only noticeable on solid dark colored backgrounds, so my first time seeing the image retention was when I opened the Kindle app with it's dark grey background, I could see a ghost image of Chrome. Most of the time I don't see this. I have a tendency to be a bit OCD when my gadgets don't work the way the should, so it remains to be seen if this will be one of those things that bothers me, it does a little, but for what I paid, I might be able to overlook it.
Another issue... I'm not quite sure is an issue, but I swear it seems like sometimes the display dims and brightens a little bit once in a while. it's pretty infrequent and quite subtle, so I'm not even entire sure if it's me or not. Auto brightness and adaptive brightness are both disabled in power options and I can't see anything else that would be causing this. I'm trying to watch for this more closely now, but don't yet have definitive proof if it's really happening or my eyes are flaking out on me.
Build quality.
Both are very nicely built products. Both have magnesium bodies. The Surface Pro 3 has no coating and has a kinda smooth but firm feel to it with just a small plastic strip for wifi. The XPS 12 has a soft touch rubberized coating on it's magnesium body and it feels really nice. But there is also quite a bit of plastic accents around the palmrest and the LCD frame. I have a refurbished XPS 12 and the plastic accents already show quite it bit of scratches and stuffs on it's edges. Previous generations of Surface tablets were very vulnerable to scratching, but with the SP3 Microsoft stayed with a natural color without using any sort of anodizing, so maybe it'll resist scratching better than previous Surface tablets have.
The XPS 12 is about and inch wider, half an inch taller, and about 25% thicker than the SP3 with keyboard attached. I forget the exact weight of the SP3 now, but the XPS 12 is 3.2 pounds and I think probably a pound heavier than the SP3 with keyboard. So it's definitely more bulk despite having a screen that is only 0.5 inches bigger diagonally.
Battery life.
It's still early, but I'm thinking the XPS 12 probably is going to get about an hour less of battery life than the SP3 does. I seemed to be averaging about 8 hours on the SP3 with most web surfing and videos. And it looks like the XPS 12 is only able to last 7 hours with the same tasks and brightness using balanced performance. The SP3 is truly impressive the battery life it gets in such a thin tablet.
Noise.
My SP3 fan ran often, but most of the time I didn't hear it and when I did, it still didn't make much noise. The XPS 12, I think has fewer fan profiles, it seems to not do anything, then suddenly roars to life, louder than the SP3. I hear this fan more on the XPS 12 than I do the SP3. And when I'm really pushing the XPS 12 it has a tornando mode. SP3 definitely wins big in the quietness department. Though the SP3 might be too quiet because I hear people say it throttles too much. I haven't really tested throttling on either device yet.
Ports.
SP3 has a full sized USB 3.0, mDP, and micro SD. The XPS 12 has two full sized USB 3.0 ports, mDP, but oddly no SD slot at all.
Speakers.
Both are good. SP3 has forward facing speakers. But I do like the side facing speakers on the XPS 12 a little more because they are quite a bit louder and seem to have a fuller sound.
Summary...
I really think the Surface Pro 3 is a very nice piece of hardware. But my problem is when I want to use a tablet I reach for something smaller like the Dell Venue 8 Pro (I really love this guy) or a Nexus 7. And when I need to do some typing I reach for my Dell Latitude or HP Chromebook with a real keyboard. The SP3 just feels like too much of an inbetween device, too big to be a go to tablet and insufficient keyboard to be a laptop. I haven't turned my SP3 on in 3 days now
XPS 12 feels a lot closer to what I'm looking for. A laptop with a touch screen and can fold down to be used as a tablet on my lap or desk, I could hold the SP3 easier because of it's weight but both are big enough I would usually set them on a surface anyway.
And the XPS 12 is attractively priced, got mine refurbished for $550, but that deal is over at the moment (will probably come again) and I see it selling new on Amazon for $680, which I think is still an enticing price despite the image retention issues. I think the XPS 12 is a very nice convertible for it's cheap price if you are able to overlook the image retention problem.
An alternative to the XPS 12 is the Yoga 2 Pro. It's display just folds back and leaves the keyboard exposed. But it has a much higher 3200x1800 display (with has it's pros and cons with DPI scaling), a bigger screen at 13.3 inches and a refurbished model with the same i5/4GB/128GB specs as the XPS 12 runs at $800 on NewEgg at the moment. Though I'm not sure what the battery life is like on the Yoga 2 Pro and I've heard it uses two possible displays, one made by AUO and one by LG and apparently some of the LG displays maybe having image retention issues as well.