Other than differences in the radio or controller settings, i don't know why this would occur only on the new models. The client should be the one deciding which radio to connect to.
I've seen this happen in our environment on older AP's as well. There are some things you can do to make clients roam more aggressively.
1. you could experiment with increasing the minimum data rate. This also effectively shrinks the cell size, so you would need to ensure there is still appropriate coverage.
2. you could try using probe suppression and force disassociation thresholds. I'm currently testing this option in certain buildings where I want to ensure clients in specific area's or rooms are connecting to the closest AP.
