3200 has a bigger/better/smoother/softer backlight. It supports 4x5 film. More professional-level software package available.
The 3170 (which I have) has a very uneven backlight, but apparently software interprets the stripes, and compensates during the scan, so the film scans show no sripes. It supports 35mm film well (up to two strips of six each), but 120 film carriers are limited... and it stops there. Each 35mm image, at 3200 d.p.i., can take up to two minutes. Comes with Photoshop Elements 2 (but no Elements manual), and costs $199.