the difference in minimal i highly doubt you will be able to notice a difference. Benchmarks are similar, overclocking potential is similar. A lot of the differences are placebo affect if there are any.
Galaxian Soup is based off of the Android Open Source Project, AOSP, which means it is as close to stock Android as humanly possible.
CM9 is a port based off of a CyanogenMod 9 ROM for the Galaxy Tab 10.1 it was originally a port but now it is an official CM9 ROM. Cyanogen has more features then stock Android. Like a heavily customization launcher (trebucket in CM9), a theming engine, and other built in tools. CM9 doesn't have as many features as CM7 but it will get there
The choice is a matter of preferences. Do you want a stock Android experience that you get to customize how you prefer, or do you want to take CM9 and use the tools they provide to make customization easier.