What kind of driving do you do, mostly city, mostly highway, or an even mix? When you do city, is it suburban type (speeds about 45mph with spaced stoplights) or urban (speeds around 30mph with a light every block)? Do you frequently have passengers and gear, or are you usually solo?
The Prius wins hands down in terms of urban-type city driving. The Prius' ability to drive with the engine off is key here, but the HCH isn't a slouch either. Head to the suburbs and it's a a draw in terms of mileage. The further the stop lights are apart, the better the HCH will do vs. the Prius. Head on the highway at 60+
mph and the HCH would most likely win. The HCH's tall gearing and super efficient engine make it a killer highway car from an efficency standpoint. The HCH will not have as robust of accelleration for the on-ramps though so if merging is an issue for you, the Prius should have a leg up. I think the Prius' extra power would also give it an economy edge over 80mph.
Ergonomically, the HCH is best car I've ever owned--everything is in exactly the right spot and it feels great. I especially like the digital speedometer and instant fuel gauge high on the dash where they are always in your line of sight. I don't care for the Prius at all ergonomically, but others love it--drive both and see what you think.
The Prius blows the HCH away in terms of interior functionality though. I miss the ability to lay my back seats down and put in long loads. The HCH trunk is also a compromise. It's fine for DINKS like my wife and I, but if you have kids or passengers, the Prius wins.
The HCH is cheaper and generally more available than the Prius, plus you would get a $525 tax credit *, whereas the Prius credit has run out already. There is only one option (nav system) on the HCH which makes it easier to find the car you want--basically it comes down to color and nav or no nav. The Prius has a huge list of options. This is great if you are ordering one, but it makes it harder to find one on the lot that isn't loaded to the gills with options (assuming you can find one on the lot).
Both are great cars and should exceed your expectations for reliablity.
* If you pay the Alternative Minimum Tax (AMT) you won't get any hybrid tax credit.