From the manufacturer:
S3
Movie functions
Powerful video functions make the PowerShot S3 IS far more than just a high-performance digital still camera. A dedicated movie button means users do not need to switch modes to start recording, so spontaneous moments can be instantly captured in full motion. Users can record smooth 30-frame-per-second VGA quality movies of up to 1 GB with stereo sound, or create 60-frame-per-second QVGA clips for sharp slow-motion playback. The Movie mode also allows users to pre-select exposure and white balance, zoom throughout the camera's focal range, and manually adjust focus while shooting. A Photo In Movie feature enables the capture of full resolution digital stills during video recording.
S5
A variety of movie-making options With a highly flexible movie mode, you can create the movie thats perfect for any application. Select from VGA (640 x 480 pixels) with frame rates of 30 fps and 30 fps LP, also QVGA (320 x 240 pixels) with frame rates of 60 fps and 30 fps for recording up to 1 hour or 4GB. With the S5 IS, seamless movie shooting is easy -- just by pressing the Movie button without switching to shooting mode. The PowerShot S5 IS supports the USB 2.0 Hi-Speed standard, so youll enjoy the fastest possible data transfer speeds when using a USB 2.0 Hi-Speed compatible computer.
The thing that's deceiving is when they say up to one hour (which can be found on the Canon website for the S3) - I think that's on the lowest quality/smallest file setting. If you leave it on best quality it's siginificantly shorter, like 8 minutes on the S3 anyway.
The question then becomes, how many minutes of video are you planning to shoot? If you want to shoot longer than 8 minutes, you should probably change the settings or plan to stop and start again (and possibly change cards), can edit later.