As the 4Gen IMA platform, the HCH is significantly more sophisticated than the Honda Accord Hybrid. Second, the Accord hybrid is not going to improve much on the FE of your TSX. Granted, you'll have the benefit of using regular gas but still, not a dramatic difference when comparing your current TSX mileage to that of a typical Gen 2 HCH.

The trully green hybrids are the HCH and the Toyota Prius as they are rated as PZEV vehicles.

The Nissan Hybrid is not a good long term bet as a platform that Nissan is willing to support. This information has been public for quite a while as Nissan intends to develop their own hybrid technology.

The Camry hybrid may be what you are looking for but sadly, it is not as green as the Prius or the HCH.

Cheers;

MSantos