First of all, let me say that I think you are approaching this difficult decision the right way. By that, I mean that you are looking at all alternatives, trying to decide what is the best decision for your family.
Second, let me say that I don't think there is a bad answer - any DVC membership, no matter where it is or how you buy it, is right for your family if it is right for your family. One of my favorite DVC quotes, which I attribute to Beca is, "The best contract is the one with your name on it."
The real question here is what is best for your family, and none of us can answer that. You have to look at the options, do the math, and decide for yourself. One caution I would give is don't rely too much on the conventional wisdom. Conventional wisdom does, in fact, represent the majority opinion (whether right or wrong), but it may not fit your particular circumstances.
"Buy where you want to stay" ("conventional wisdom") fits many DVC owners to a tee. It is the only rational choice for many. It is critical 1) if you usually can book more than 7 months out and can therefore enjoy the 11 month window, AND 2) if you CARE where you stay. It is especially important if either a) you expect to visit WDW during peak DVC demand periods (which are different from peak WDW periods), and/or, b) if you need specific accommodations like an OKW Grand Villa.
Classic examples would be the family who want to go to Food & Wine at Epcot and stay at BCV/BWV, or the family who wants to go to MK during Christmas season and stay at VWL. Those are not the only situations, but they are classics. If that description fits your family's needs, and you can book 7+ months out, then "buy where you want to stay." It is your only sensible choice, and resale will most likely offer you the least expensive way to accomplish that.
If you can't book 7+ months out, the above has zero value for you because "...points is points" at seven months. If you don't travel during peak periods, the above has limited value for you and other considerations may outweigh the 11 month window we all hyperventilate over. The truth is, most of the time you can get what you want at 7 months unless you are going at peak times or want something really special.
If "buy where you want to stay" is not dictated by your family's needs, then your calculations become much more complex. I suspect, from your original post, that you are more flexible (as we were) and therefore have more options, but also a more difficult decision.
If that is the case, your real decision is whether you should buy SSR, with its incentives and financing and 49 years worth of points, or buy resale and enjoy a possibly lower entry cost.
I can make an argument that - considering the long-term cost of ownership - buying SSR is actually a little cheaper than buying just about anything resale. But I won't. What I think you will get from many sources is that the cost is about the same, and it's really a question of what fits you better. Buying SSR direct, you pay a little more for a little more; buying resale, you pay less for less.
In my case, I bought OKW resale for $73 per point (a price lower than many are currently paying). I got a good contract and am comfortable with my decision. However, I had some false starts during the process, and if that contract had fallen through, I would have bought SSR direct and been perfectly comfortable with that decision.
The advantages of resale are lower initial cost, lower initial cost, and lower initial cost. Sure you can select your resort and use year (to some extent) but you can do the same buying direct from Disney. The real difference is lower initial cost. Another advantage is that you can sometimes get contracts with less than 150 points per year. You can't do that buying direct.
There are several negatives to buying resale. The procedure is complex, frustrating, and unreliable...and that is on its best day. There are three key points at which it can fall apart - seller not accepting your offer no matter what it is, Disney ROFRing your contract, and seller backing out at the last minute. People often focus on the resale brokers, wondering if they are legit. They're legit; don't worry about them. The people you need to worry about are unscrupulous sellers and the Disney ROFR folks.
With resale, you usually pay closing costs, and that is not a trivial expense. They seem to run from a minimum of about $400 up to about 3% of the total sale.
With all resales except SSR, you are buying 37 years of points vs. 49 years of points with SSR. Some will argue that is inconsequential, but they are incorrect. To be precise, that is a 32% difference. Even if you don't intend to keep your DVC for 49 years, those extra 12 years have serious value, and may make a big difference when you sell 12-15 years down the road.
Another disadvantage of buying resale may or not be a factor for you. Buying resale takes a LONG time. You should figure on 8-12 weeks for the process from start to finish - from your original offer, acceptance, through ROFR, through closing, and finally "in the system" and able to actually use your contract. You may get lucky, and there are things you can do to shave 2-3 weeks off the process, but no matter what you do, it is S...L...O...W. And because it is filled with the uncertainties outlined above, it can be very stressful.
The advantages of buying direct are not subtle.
You are dealing with Disney, not someone you don't know.
The process is known, it is quick and painless in most cases, and the eventual outcome is never in doubt. When Disney gets the points, you have them. If you are buying a resort other than SSR, the wait list can be excruciating once in a while, but the outcome is known and ROFR is guaranteed to be excruciating and uncertain every time.
And finally, Disney finances. That is important to some, not important to others.
The only real downside to buying direct is you will pay more. SSR is currently $87 per point with incentives. Direct resales are $89 with no incentives available. We paid $73 per point for OKW, but others have recently paid $78-80. BCV is about $83-84 to pass ROFR, and BWV was ROFR'd this week at $80. To all of those prices, of course, you have to add closing costs, time and mental anguish.
To close, I come back to the earlier statement - there is no bad answer if DVC meets your family's needs. "The best contract is the one with your name on it."