Depends on what the lenses you mostly use are. The A75 has a fairly restricted zoom range - about 35-105 mm range 35 mm. equivalent. If you like wide angle work, or longish telephotos, you'll be better off going with something other than the A75. Maybe the 300d, bearing in mind that there's a 1.6x multiplier on all the lenses so an 18 mm. will give you 28 mm equivalent, and a 135 gets you better than 200 mm. equiv. You also need to remember that the 1.6 x multiplier throws off the old 1/focal length rule of thumb for safe hand-held speed - you need to use the corrected focal length, and may want to look at getting Canon's IS (Image Stabilized) lenses for anything beyond "normal".
The A75 has a pretty good out-of-box "macro" capability, which you can supplement with close-up lenses. Of course there are close-up and macro lenses and extension tubes for the 300d, as well.
I also find that using the panoramic stitch tools can often supplant use of wide angle lenses for landscapes, but not for anything with fidgetty subjects.
Both of the cameras will allow manual control, so you're fine there.
Actually, if you're looking at the A75, I'd move up a step and check the A80 instead. The back LCD screen will swing out and pivot so you can use it at odd angles, and that's quite handy at times. It zooms to 38-114 mm equiv. (where I fall back on panorama stitch), and is a 4 Mp camera, but otherwise, they're quite similar.
The Digital Rebel/300d will give you more flexibility for growth, even if (and I don't particularly recommend this) you start with the 18-55 (28-90 equiv.) kit lens. It lets you use external flash as well. The 6 MP will give you better resolution, too.
The PowerShot Pro 1 has TTL viewing (on an LCD viewfinder) and a 28-200 equiv. fixed zoom lens with 8 MP total, but I think I'd stay with the 300d in the same price range. You get more flexibility, and the smaller sensor with higher pixel count in the PS Pro has some disadvantages with image noise..
I made the jump about 2 years ago, giving up my 3 Canon FD-mount cameras and ?? lenses for an Eos D60 when I realized I'd been using my Kodak DC260 and DC290 in preference to the film cameras. I've since added an A80 as a small, take-with camera and find the combination very good (although I'm still drooling over the Eos 1D Mk II).