if u want to choose between the SD100 and A70, i'd say it depends what your looking for. if its size, then the SD100 is best.
SD100 is 2x Optical Zoom
A70 is 3x Optical Zoom
(don't worry about digital Zoom, it's useless) unless u want pixelated pics.
SD100: has no Manual Focus
SD100: proprietary battery (expensive)
A70: (4 x AA) easier to come by
SD100: cannot accept Wide/Tele Lens
SD100: SD, 256 @ $114
A70: CF (Compact Flash), 256 @ $85
Since the A70 has a smaller aperature of 4.9 (smaller is the higher number) means it will allow you to use a slow shutter speed on a bright sunny day. slow shutter speed allows you to depict motion.
Another advantage of the 4.9 vs 3.9 is the increased DOF (depth-of-field). it will allow you to take landscape pictures where the foreground and background will be more in focus.
The only advantage the SD100 has is its portable size. other than that, i would lean towards the A70.
the A70 may seem ugly, but i've held it and its very comfortable to hold. feels like a camera. the battery holder that forms the mold gives your hand a nice place to grip.
hope this helps....