I would go definetly for the Cat 4 / Poly combo. I think with 2 kids under 5, you'll appreciate the 'cost' of convenience. The monorail is nearly worth it alone. And, I spent many afternoons on the verandah of my cat 4 when my DD was sleeping. I would not have been happy to spend the napping time stuck in a dark cat 10 watching TV while DD was sleeping. For me, the verandah and napping time was a great alone time for me!
We spent a day or so at the Poly and I wish I had MORE. It was great to not worry about bus / car transportation to MK.
If the kids were bigger my choice would probably be the 2 cat 10s, but for now, I'd go with the verandah.