For us (my kids were the same age as yours on our last trip) it was a must. Yes, the transportation was a time sucker. Yes, I would have rather been at the parks. But having a pair of overly tired, whiny, crankey kids was simply not worth the price.
Unless you have a REALLY good stoller napper. I think a break is a must (especially if it is hot) and you may even find that *you and dad* need a break as well.
I would recommend you maximize your time in the parks by 1) being there before rope drop, no exceptions

2)have a good touring plan and stick to it (I love RideMax, because it allowed me to tell my oldes exactly what would be happening when 3) plan your table service meals VERY wisely. We opted to only do the character breakfasts - I knew kids would be hungry then, it killed 2 birds with one stone (who wants to wait to meet pooh when you can chat it up over breakfast). If possible, choose a time before the park opens as you get great pics and do not miss park time. Our other meals we settled for counter service. Yes, it got old but it allowed us to eat when kids were hungry and it minimized the time we were not "on the go" in the parks.
Also, if you stay up later than the kiddos, consider a "free pass" at night for one parent. Let dh head off to dtd or back to the parks while you stay with the little ones and the next night you switch. It is not as much fun as if you are together, but that my be your only chance to catch a show in peace or watch illuminations, or even ride Space Mtn!
Have fun!