While both Simplify and Impression are art driven programs, there are different technologies behind each.
Topaz Simplify uses size-based technology (a scale-space algorithm) to remove realistic details from your photo, resulting in a more graphical or flattened image.
Painting effect in Simplify
With Topaz Impression, actual brush strokes were scanned into the program's algorithm to produce the most realistic painting and other media results possible.
Painting effect in Impression
So in a nutshell, Simplify produces more graphical art results and Impression creates more realistic art results. One feature to not overlook in Simplify is its Edge technology, which allows for the creation of line & ink sketches.
Edge technology in Simplify
This effect cannot be reproduced in Impression. On the other hand, Impression contains textures (something Simplify does not offer). The only way to truly understand the differences is to test each program out for yourself.