I find it funny that every is talking about a day at US as "taking away a day at WDW." I can't see her choice in those terms.
Isn't OP choosing how he WANTS to spend her time? Isn't she CHOOSING the most appealing option?
Wouldn't she be ADDING an ADDITIONAL experience to his vacation? If she decides to spend her entire vacation in ONE place, isn't that the option with LESS variety? Mind, WDW has enough entertainment to keep folks busy for a full week.
(I posted this before reading the 3rd page.) Since she asked the question backwards, I gather she'd already decided what she WANTS to do before she posted, and is mostly seeking validation of her choice to SKIP US. (YUP!)
That said, her kids are plenty old enough to enjoy US.
Further, US prices aren't the problem, IMO. The problem is WDW's high hotel prices, on a day when OP wouldn't be using the WDW perks. In other words, why pay for the WDW buses on a day you aren't using them?
OP would do much better to modify his WDW hotel reservation, and make it one night shorter. Cancel TWO WDW park days off her tickets, spend one night onsite at US, and TWO days exploring US.