We've only stayed at Pop Century, so I can't compare it to the others. But, we really liked it. I'm sure the theming isn't quite as young kid friendly as the All Stars, and I don't think anything can compare to the kid-centric theming at the AoA (we are totally going to explore that one on our trip also, if I can convince dad.
).
But, we stayed at the far reaches of the resort both times and didn't find the walk too difficult (we prefer the outskirts of the resort for the quiet). We were in the 90s section the first time and could easily trot across a parking lot to the buses. Very quick. But it was much warmer crossing all that blacktop.
Our last trip we were in the 60s (I think), on the back side facing the lake and AoA. A slightly longer walk, but still not bad.
We found the buses great. We never waited long. The longest drive is to MK, so that's not ideal, but still not awful when you had a seat.
Just do NOT get on a bus back to the resort at the end of a long day if you will end up standing with your little guy. Just my opinion, but it would be easier to wait for the next bus (which come quickly when the crowds are exiting the park) than to try to stand with a young one for the entire ride back. (We actually try to follow the advice of staying seated while the crowd exits, then heading out as things quiet down.)
Good Luck making the resort decision. That's a tough one!
As for tickets. I think you should get the 4-day pass. You can always add a day to your tickets if you decide you want to add another park day.
We're going to be at Ft. Wilderness for 5 days, but we're only going to have four park days. We'll just enjoy our resort, DD, or maybe some putt putt on our check-in day.