Well I just couldn't wait. I went over to a store that stocked the Ergo. I tried it on and it seemed much cooler than the HAB. Poppy went to sleep straight away, which was another big selling point.
The baby can't face out in the Ergo Sport, so we'll still get some use out of the HAB. Plus it's probably better for my back to have the two, which seem to distribute the weight differently across my back.
Another bonus of the Ergo is that Poppy seems happy in it when I sit down. In the HAB she'd start crying if I sat down. She must get squashed somehow.
So, one hour after buying the Ergo I'm thrilled!