Well.....I've owned both of the cars you've mentioned, so I think I have a good perspective on them. I had a 2004 Mustang GT (same as 03) and a 2001 Firehawk (similar to camaro, almost identical to SS)
As far as "fun to drive", I think the Mustang was more fun to drive. You could basically drive it sideways everywhere - it was predictable, lively, and entertaining. It wasn't all that fast (fast enough though) but it sounded great.
In terms of outright speed and handling, it was hand down the GM. The firehawk was significantly faster and had significantly higher handling limits. On the track (road course), it was a much better car because of this. For the street, though, it was bulky feeling, the seats were oddly low-feeling, and it was much less predictable (more than likely because of the power). It also didnt sound as great as the mustang, more than likely because of the y-pipe configuration as opposed to the H pipe (or X pipe after I added it) configuration of the mustang.
The GM motor felt much more high-tech and snappy, and aside from the power, it also drove better (the mustang is very slow to come down from revs, and also has annoying whiny power steering/brakes like all fords do). Funny because the OHC motors are supposed to be better. The GM motor had torque and power EVERYWHERE.
I dont think you can go wrong with either. For daily driving, I would get a mustang - for modding/racing, I would get the Chevy. I am an amateur race driver so for me track performance is usually #1, so take that into account in my opinions.
I ended up selling both of them and getting an 07 vette, which gave me the GM power but matched with a more sophisticated chassis that is far better on the street than the camaro/trans am (better on the track too, but not WAY better as some would like you to believe).
Hope that helps.
