I have an 04 Prius so I'd have to say go with the Prius, I've thought about the Camry at one point but the Prius is considered a mid-size sedan! I have the HOV Carpool lane stickers (no longer offered in CA) and that was the main reason for the Prius.
The interior space is just as roomy as a Camry; you wouldn't think of it as being roomy just by looking at the exterior. YES, I have confirmed that the trunk space is good, also, if you remove the flat level floor board, you will received additional trunk space.
What you are looking for is MPG! That's the whole point of the Prius. On occassion I get 50MPG on flat highway roads. Daily mixed driving I get about 48MPG. When I drive with a lead foot 70mph+, I get 45mpg.
Also, filling up the Prius at the pump is where I get the most enjoyment. Here in Norcal at about $2.89 a gallon today, you'd fill up for about $20 now. When gas prices dropped several weeks ago, I have paid about $16 for a full tank.
FYI, your range will differ depending on driving style. I have gotten 440 miles to the tank babying the gas peddle, driving in the slow lane. BUT for normal everyday driving I get about 350miles to the tank. You get consistent 50 or even 51 MPG on flat highways.
Good luck!