I've been spending a lot of time with both of them lately - sort of ping-ponging back and forth between them - and I'm more struck by the differences than the similarities.
Not a direct response to your question (I'll try to get to the itemized list below), but the main thing that stands out for me is that they feel very different.
The DC-6 is very clearly a PMDG product. It's very formal. The flight engineer executes flows and checklists, and as pilot flying, you're relieved of responsibilities like handling the throttles (except on the ground, takeoff and short final) - which is in fact SOP for a complex multi-engine prop (PF calls manifold, FE works the throttles - you can see this in action in the YouTube video of Fifi landing at Oskhosh). Textures are photo-real, or give that impression.
The Connie is, for want of a better term, warmer and more artistic. The FE mutters to himself as he works, which keeps you current on system status, but the side effect is it feels more improvisational. And as noted, the Connie includes the cabin simulation (with lots of things that can happen), the career simulation, richer interaction with the environment (variable engine starts, window fogging, cabin temperature as an important and consequential thing to manage). Sounds are used very creatively to add to the immersion. Textures are hand-drawn and can feel a little lacking in detail, which is why the community likes to produce enhanced "worn" VCs for A2A aircraft (there are several for the Connie). Their best VC by far is the one for the Cherokee. The Connie isn't quite at that level, though it's better than the Stratocruiser, which could use a refresh.
The Connie can be flown with modern avionics but it feels a bit compromised that way (you can use a modern autopilot with altitude hold, but it's the default FSX/P3D one). The Sperry autopilot is quite limited - best used in level flight only. As Tim notes, it works best as a period aircraft.
The DC-6, on the other hand, feels right in modern airspace, though if you drop the GPS you can fly it in period style, albeit with updated radios. It's a later design with a more advanced period-authentic autopilot that can hold altitude and track a VOR, localizer or (if you have it) GPS course. It's also quite a bit faster than the Connie. The pace of development back then was such that a few years made for a more capable airplane.
I much prefer the flight dynamics in the DC-6. Both flight models are good but for me at least, Alexander Metzger has a way (as Rob Young did) of giving a simulated airplane a real sense of mass and inertia. It feels substantial. The Connie is a bit twitchy in the pitch axis - though that's characteristic of the real airplane, so some of what I'm expressing here is a personal preference.
Another personal note - I don't much like the voice acting on the Connie. I loved the voicing on the Stratocruiser and B-17. The actress who did Heidi for the Stratocruiser and the Cub had a brash, brassy 1940's quality that was exactly right (think of May Wynn and other Herman Wouk heroines and you'll get the idea). For the Connie, A2A turned more to family members, and while I'm sure they're very nice, gifted people, they're uneven as voice actors. Betty (voiced by Scott's daughter, I think, so I say this hesitantly, but it's how I feel) sounds young and modern - I keep expecting her to snap her gum in my ear, or take pictures of her food and post them to Instagram. Unlike the avionics, Betty fits better in modern repaints - perfect for charters to the Caribbean, not so much for 1940's line flying.
I agree with Tim about the startup on the DC-6 feeling much too scripted - that's the weakest element about the DC-6. While an A2A style full engine system simulation would be wonderful, I'd be OK if PMDG would just loosen up the scripting a bit and add more variability, so that the start or failed start didn't invariably happen at 12 blades, which seems to be the case at the moment.
My bottom line is - all my quibbles are minor, and they're both so good I wouldn't want to be without either one. If I had to pick, I'd lean a little more than the DC-6, because I'm not sentimental about the past, and because, while I love the idea of the Captain of the Ship career module in the Connie, in practice I wind up not using it. I spend enough time in my real life getting evaluated, and usually opt out of it in the sim. Also, the career parameters sometimes feel more GA than airline oriented (e.g. points off for a hard landing, even though at times planting the airplane firmly might be the better, safer option). Again, these are very personal notes - a classic case of YMMV. When I see the Connie, I decide it's so beautiful that that's the one I want to fly - how could you choose anything else? My real advice is - they're wonderful. Get both.
As for specifics:
1 and 2 - covered above.
3 - I'd give the edge to the DC-6 for external modeling. It's extraordinarily good. The Connie is no slouch, but the DC-6 is a bit more detailed.
4 - As noted, for the VC I'll give the edge to the DC-6. The detailing in the textures makes it feel more like a real airplane, to me at least. But the Connie has better environmental interactions - window fogging and such.
5 - Sounds - advantage Connie - the DC-6 is good but the Connie sounds are a "you are there" experience.
6 - Documentation - equally good though very different. The Connie manual is a beautifully produced book with an extensive history of the development of the airplane. There's good operational detail, but you might want to supplement it with a period POH. The DC-6 comes with a complete POH as well as two detailed tutorial flights (with a third on the way).
7 - Additional features - a tie. I don't use Captain of the Ship, though I might get around to it. I do use the automated cockpit crew in both airplanes. To me, that's more realistic than trying to be a one-man flight crew (plus, I hate micromanaging). They're both good.
One more consideration - the DC-6 is already profiled in SimBrief and PRO-ATC/X. For the Connie, you'd have to roll your own profiles (not hard to do). That may or may not matter to you. Noted for what it's worth.
Both airplanes are absolutely worth the price - no question.
Again, all of this comes down to personal taste, and at the end of the day what we're talking about is a choice between two of the best aircraft ever developed for a flight simulator.
Hope this is helpful.