I had a Rally Twin and adored it.
I read your other thread, so I'm up to speed with what's going on.
Anyway, while I don't love the Quattro Duo, I'm thinking you should just hold onto it for now? You never know, but your middle ds might settle into the idea once the new baby is here. And honestly, with 3 kids and a liking for the Duo, I'm wondering if going from a QD to a RT might be too big of a leap? It's a totally different stroller animal. Not car seat compatible at all, and none of the creature comforts a lot of people appreciate (cup holders, enormous basket, etc).
I dunno.. If you've liked your QD up until now, and it's ds that's forcing this decision I think I'd hold off... I've made some toddler inspired stroller decisions lately, and well, let's just say I really wish I'd stood my ground.