As a true "Pontiac bigot", of course I would recommend the Firebird, but... The '78 Z is a decent place to start, too. If the Firebird is 100% intact, and only needs minor resto, it's the better deal. The Camaro is going to need more than what's listed. My experience has always been that a seller with a modified vehicle, often either doesn't KNOW what it needs, or will not provide a "full disclosure" of problems.
Look at the Camaro. If it looks like the people that made the changes were consciensious (attention to detail, no billy-bobbin') and installed the appropriate safety equipment (cage, fire extinguisher, harness, etc.) to go along with the go-fast goodies, it could be a good deal. If it looks like a hack job, avoid it. Also, if it HAS been extensively modified (the chassis and body, not the power train), resale value suffers drastically as a "collector car". The late '70s Z/28s aren't exactly worthless.
The Firebird will have good resale value down the road. TransAms are getting "used up", so, like LeMans and Tempest (GTOs are either gone or EXTREMELY expensive), Formula will float upwards as Pontiac people look for new meat. The Pontiac will also show more performance potential in street trim than the small block Camaro (big block cars are a whole different animal, and animals they are!). Tuning a small block for 500-plus streetable horsepower is easy enough, but the price is high. The BBC and Pontiac both offer much better low-speed power potential, a key ingredient for a drivable "heavy" performance car. These cars are in the 3,800 lb. range when fully equipped and a driver.
ALL the "good" suspension pieces from the WS-6 T/As ('77-'79) will fit either of these cars, too, making them lethal in the corners.
So, flip a coin, do "rock/paper/scissors" or other "which one?" game. Both cars represent an era that has gone by, but still can be reckoned with in the modern "performance world".
Jim