You're talking apples and oranges here. They're entirely different apps. WisBar modifies your Start Menu into a cascading format, among other things, and gives you the option to put quick start icons on your Today screen. Check
www.lakeridgesoftware.com for additional. Mobile Shell gives you a Now screen with the time, weather, and calendar avaialble at a glance. It also provides panels for speed dial, frequently used apps, and weather ( I think). It will also put a plug in on your Today screen so you can put various apps in tabs to free up space on the Today screen.
www.spbsoftwarehouse.com
Bottom line...the programs do not duplicate each other, so you could easily run both. BTW, this probably belongs in the Software forum