I would say the best way to describe a Pontiac V-8s is not by its physical size, but by the main journal diameter- 265/301 short-decks, 326/350/389/400s all have 3.0" mains, and 421/4428/455s have 3.25" mains. As mentioned earlier, all Pontiac V-8s are about the same size externally, so the only real way to tell the CID is by checking for a casting number (i.e. "400" indicates 400 CID) under the head, and block serial no. stamp.
I've owned both Chevys and Pontiacs, and both have their strengths and weaknesses- the best thing I could say about both is they're typical GM, keep them maintained and they're reliable. I personally prefer the Pontiac Firebird when it comes to F-bodies- better overall styling and handling IMO. A Firebird is probably a better investment, since it's lower production numbers and high popularity increase it's value.
Geno