I have both, the slingshot has a little more space (wider - top to bottom of camera section) in the bottom section but is tight for extras in the top section. It does however have a really good swing it round the front access system so you can get at your camera without fear of it dropping out.
The fastpack is much roomier up top and can still take a 50d(gripped) with a 28-135 attached, a Canon 17-55 f2.8, a Sigma 10-20 f3.5, Canon 55-250 and a 430exII flash. in the bottom section.
If its just for day to day camera transport and use then get the slingshot.
If you are doing a lot of trekking and you need room for a waterproof, compact camera, drinking bottle, sandwiches etc then get the Fastpack.