Some thoughts:
1. The Annual Pass gives you theme park entries for a year. The Premium Annual Pass also gives you unlimited entries to everything else (including Disney Quest)for a year. That PAP, in other words, provides the same thing as the length of stay pass except it does it for a year rather than just for your 14 day length of stay.
2. The PAP without a discount costs less than a 13 or 14 day length of stay pass without a discount. As a
DVC member you get a 10% discount on the LOS which would make it cheaper for your time period than the PAP, but if you are a Disney Club member you would get about a 5% discount on the PAP which would bring it in close to the same price as the LOS with the 10% discount.
3. Now, when you to renew that PAP it will cost significantly less than when bought new. Thus, during your second year (and third year, etc.) with a PAP, you will likely pay about $40 to $50 less per person than you would for that length of stay in the second year (and subsequent years).
4. What I am suggesting is this: you have absolutely no reason at all to buy the length of stay pass because you can do it cheaper with the PAP which is better than a length of stay.
5. However, I am not suggesting that you buy the PAP. You probably do not need it. If you buy the AP and go to waterparks (or PI, DQ etc.) 4 times or less while you are there and pay single day entry fees for those, the combined cost of the AP and those entries is less than the PAP (and less than the LOS). In other words the PAP makes sense only if you are going to do 5 waterparks/DQ. But note, also, that as a DVC member you get 10% discounts on waterpark single day tickets and 20% on DQ. Because of that, you really need to do waterparks/DQ more than 5 times before the PAP makes sense.
6. Thus, it boils down to this: if you are going to do waterparks/DQ 6 times or more buy the PAP. If not (which is highly likely for most people) buy the AP. Get a Disney Club membership first to get the discounts on the annual passes. You have no reason to stick with the LOS if you go for 2 weeks every year. The annual passes also give you some restaurant and merchandise discounts but many of those match the same discounts you get with the DVC membership.