Thank you everyone for taking the time to offer your input. So many good points and suggestions!
We want to buy direct so that we can use points wherever we want for the next 40 years. We are in this for the long game, but if we ever downsize our contracts, it will be our 50 Poly resale that goes first. We can see a case for resale points, especially BWV or BC, but after 2042 we would strongly prefer to have direct vs resale. So at WDW that really does leave it between Poly and RIV (CFW we can not justify owning although we'd love to stay there).
Our two favorite resorts are Poly and RIV. As a family of three, Poly longhouses have worked out great. And we've also stayed at BLT, RIV, BC, AUL, VB, AK, and SS. As our tween gets older it would be great to have options for larger accommodations, so our home resort of Poly adding those options seems like hitting the jackpot (well almost). It seems to make sense to add on to Poly so we have the option to stay at studios or larger and then if we want to try to switch at seven months we can. If we get "stuck" at Poly we will always be happy.
Since we love RIV too it would be great to have home resort advantage for RIV studios. But we'd be okay with preferred view if standard is not open. The RIV one bedrooms are gorgeous (we've stayed in one) but the shower situation is actually better in a RIV studio so the Poly longhouse studio or one-bedroom is better for us. RIV two bedrooms would be amazing but we would not need them unless maybe our kid's friends were travelling with us. It is expensive to buy Poly but we are already in for 250 points and don't need to add much to really expand our options. We wonder if adding on at RIV for studios would eventually encourage us to buy even more points than we would if we just added on to our existing Poly. From a practical perspective it seems to make more sense to add to Poly. It would be easier to manage all points at the same resort. There is some RIV FOMO, however. But I think we can switch into RIV enough to make us happy. And, we enjoy staying at all DVC properties.
And as foolish as this may be... we want this to be the last time we add on!
Thanks again!