Hi....we just returned yesterday from WDW and UF/IOA. I had a reservation at the Dolphin at a discounted rate, but at the last minuted decided to save even more and book through Hotwire. Ended up at the Royal Plaza in DD ($58/night). I thought the DD bus transportation was misleading and because of this, would probably never stay in DD again. The buses did not pick up or arrive when they were supposed to. After waiting for 40-50 minutes to be picked up from MK, the drivers would still get off the bus intermittently when stopping at the hotels to have a quick cigarette.
I have to admit that I was very envious of Disney's transportation. The buses have the most convenient drop-off and pick-up areas near the resort entrances, and they run much more frequently.
Although I've never stayed at the HIFS and can't vouch for their bus transportation or accommodations, my vote is for the Contemporary. If an on-site hotel is in your budget, I'd stay there.
You can reach MK or Epcot easily by monorail from the Contemporary. And....bus transportation to MGM or AK has got to better from a Disney resort than a non Disney resort. There were many children staying at the Contemporary, so I think your kids will have a good time; my DD (9 years old) kept telling me she would have rather stayed there. I can't remember what the pool is like at the Contemporary, but my guess is that it's nicer and easier to reach (time-wise) than an off site hotel for midday breaks.