I use to blindly follow the "walking distance rule" too--until one time I didn't and I actually found (gasp) that I liked a shuttle better! If the shuttle is reliable and nice, it can actually be better than hauling your tired body (along with the masses) down Harbor. It makes life easier and saves your feet--which are already over worked and overtired from all that walking!
The problem with the motels across the street is that they are very motel-ish. If you want an actual "hotel", then you may need to stay further away, which isn't all that terrible, I swear! Been there, stayed in *that* motel!
As far as I know, the Sheraton that looks like a castle actually has it's own shuttle--which is like heaven on earth in my book of disney area hotels! Haven't stayed there, but likely will some day. The Sheraton Park looks very nice too--and I know it is a bit closer than the castle one. I would say if walking and being close is important to you (as well as easy access to McD's), then go for FFI, and if you want a more "hotel" type vacation with all that includes, then go for the Sheraton.
Good luck and let us know what you decide!