If size matters, then I'd recommend the S400 over anything in Canon's A series. The advantage (in general) of the A series over the S series is more manual controls.
Not only manual controls, which are an absolute must if the person you're buying for enjoys photography, but the A70 also has a lot of lens add ons you can buy for it (like filters, wide angle lenses, etc).
If you're buying a digital camera for someone who uses cameras to take pictures at weddings and family get togethers and stuff like that, then you may be okay if it doesn't have a lot of manual controls. However, if you're buying this camera for an aspiring photographer or someone who you think may use it a lot, then they may get bored without manual controls.
The A80 is nice but it really doesn't do a lot more than the A70--there are just some incremental improvements (higher quality CCD, possibly a better lens, etc). The images will be slightly larger but it also requires slightly more memory per shot thus fewer shots on a card. You're getting a camera that's a bit better but you're also paying an awful lot more, comparatively (about $100 more). If the extra money isn't a big deal then go for it. The reason so many people swear by the A70 is because it is truly an exceptional deal. Remember: you're going to have to add quite a bit to the price of the camera before you get a whole package price. You'll definitely want a larger memory card (128MB = $45) and you may want an extra battery ($40).
The best way to make the correct choice is to read about the more popular cameras at places like http://www.dcresource.com
. They both have good reviews and tons of pictures; the advantage of dpreview is that they also list what cameras they consider "very good" for their price range so you can weed out the field a lot that way.