Onsite or offsite never matters to me- what makes the choice is the details of the trip, the number of travelers, the number of days, the focus of the trip. In your case, I'd stay at AllStars. However, we NEVER get the Dining Plan, even when it is "free." We have used it several times over the years and I've tracked the receipts. Having the dining plan ALWAYS costs us more, like around $200 more, than paying OOP for our food, and we get to eat the way we like rather than the way the plan demands. If you want your food prepaid, figure out about how much you'd spend and then buy a Disney Gift Card for that amount, and use it to pay for your meals at Disney (and if you buy the gift card at Target using a Target Red Card you'll save 5% on the cost of the card).
ETA: In your case, being just you, DD, and park-focused, I'd stay at AllStars. Book the room and track the rates, rebooking if a deal comes out, OR find a
travel agent to do this for you; the good ones will babysit your reservation and rebook if there is a sale/price drop.
Funny story...DD31 and I stayed at the Poly last summer, resort only trip with deep discount on the price but still paid too much. Our flight home was canceled due to weather so we needed another night at a hotel- but not at PolyPrices!! We booked at ASMusic and as we were walking to our room, DD said, "Is it wrong that I feel more comfortable and at home here, than at the Poly?" We love staying at the AllStars!