I think Valet Parking at Poly is up to $33 these days. MK parking is $22. So you'll have to pick your poison.
Both seem valid to me. I can see how it'd be nice to have your car right there at Poly after the Luau. But it's not a terrible walk over to the TTC if you want to go that route.
Poly Valet would be your only option to leave your car there all day. If you do this, just take the Resort Monorail to MK.
If you park at the TTC, the Ferry and monorail should be running. Sometimes it's just the Resort monorail and not the Express at that hour, but that's not the end of the world. If neither the Ferry or Monorail were to be running (rare, but it happens), there would be buses. Personally, I'd over-allocate travel time - the worse thing to happen is you arrive at the MK entrance way early and wait for awhile. It's better than running late and the stress meter instantly going to high.
Hope all of that made sense.