I think that - when push comes to shove - this has to be more than a financial decision.
Think about the on-site off-site difference. Most people who make that decision purely on finances end up staying off site. But there are those of us that feel that on-site is actually a great deal - and that given you don't need to rent a car or you get access to EE or whatever, that it ends up being a real value.
DVC is close enough that most people who stay at Deluxes (and even some of those staying at moderates) will break even, but like the off-site on-site debate, its hard to compare apples to apples. DVC is going to really, seriously change the way you vacation at Disney (and I didn't really believe this until I got back two weeks ago). Putting the kids in their own bedroom, having easy access to toss in wash, being able to scramble up a few eggs effortlessly. Not to mention the assurance that you will be back in a couple years - so there is always next trip - will give your vacations a casualness.
There are those here that believe that a DVC room is superior to anything you'll find at a Deluxe - and those disappointed that DVC rooms aren't superior in every way (i.e. plastic deck chairs or no access to concierge or lack of daily maid service). I think it depends on what you want in a "superior" vacation. I know I'm not regretting our purchase.
At the same time, DVC is making a committment. Renting two regular hotel rooms, you can just decide not to go if money gets tight. You can go somewhere else if the family decides "Disney again?!" Once you buy DVC, you are committed (and perhaps commitable) - you can rent out your points (and do more than break even) if money gets tight or the family would rather go to Cancun, but that may be too much of a hassle.
We do the every other year thing and it works great.