I recently stayed at Poly (Dec. 8-12, followed by BC from Dec. 12-16) and it was my first deluxe stay after staying previously at a moderate (CSR). We had 35% AP discounts and that helped.
Overall, I didn't think the resort was a bust, don't regret staying there, and would consider staying there again (FWIW, I slightly preferred it over BC).
With that being said, given the price tag (even with discounts), there are a few caveats to not being disappointed:
(1) You must love the theming and overall ambiance of the resort (which we do and it sounds like you do too).
(2) You must accept that a big part of what you're paying for is location (and Monorail access).
(3) You must not expect Ritz Carlton/Four Seasons resort/room quality and service. That being said, I wasn't disappointed with the rooms (we were in Tokelou). Yes, if you took the time to look closely, you could observe some normal wear and tear. But overall, I was generally pleased and found the rooms, in addition to being larger, to be, overall, more luxuriously appointed than at CSR (and our room at CSR was perfectly fine).
(4) It helps if the Monorail is not down. IMO, there's no more magical way to start a day at Disney than to walk through the beautiful Poly grounds to the GCH (LOVE the smell in there) and, from there, hop on the Monorail. But when the Monorial is down, it's a major bummer (even with other transportation options).
(5) It helps to take advantage of some of the Poly's special perks such as Wishes on the beach, evening walks to the light of the tiki torches, and being able to walk to the TTC to catch the Epcot Monorail. And take some time to just chill in the GCH.
(6) It helps if you really like the Poly's dining options and have ADRs with them (we had a dinner at 'Ohana and a breakfast at Kona).
(7) Going back to the Monorail theme, it's really nice with young kids and strollers, especially if you stay in the MK until closing and a kid falls asleep in the stroller (don't have to fold it up).
All that being said, from a strict bang per buck standpoint, the moderate resorts are, IMO, a better deal. But I think if you can afford it, a stay at a deluxe like the Poly is worth splurging on at least once.
In summary, I think you've picked two good resorts and will enjoy your split stay (just don't expect Poly to blow CSR out of the water, especially if the Monorail is having problems).