I ended up getting the
Outback Solo for my SP3. At first I was afraid it would feel like a purse, carrying it around. I didn't want that. But no, it doesn't. Like the review on the blog said a while back, if Indiana Jones were to carry a Surface Pro 3, this is the bag he'd use. And since I'm a fan of Harrison Ford not only for his filmography, but also for his aviation, as well as the way he lives (he gives kids airplane rides, and has used his helicopter in rescues), that's the bag I went with.
I've probably had it for a month or two now, and I really like it. I keep a Microsoft Arc Touch Mouse with my PC, so the USB dongle is always attached (not the BT version). At first, the bag didn't like that, but it's broken in now, and the PC with dongle slides in and out just fine. The smaller front pocket fits the mouse perfectly, leaving the larger pouch for whatever. I have a small 4-port USB hub in there right now, but there is room for more. I haven't tried the power supply, and kind of doubt it would fit, but maybe. It would definitely be tight closing the magnetic clasp on the pouch. The pocket on the back fits my Dell Venue Pro 8, if I ever want to take both computers with me. That's going to be rare, but it's nice to know that if I want to take it I won't have to move to a bigger bag.
I'm perfectly happy with it.