The key difference is that you can't deploy application code to Streaming Server. If you need to have any application logic (which you will for text chat), you'll need an Interactive Server license.
There are major difference between the two editions you are mentioning but most important difference in your context is - No Remote SharedObjects and No Server-side coding is possible in Flash Media Streaming Server. So you won't be able to write full fledged Video Chat or Text Chat - you migth have to use some other technologies so that you can create chat rooms and use FMSS to do Video Streaming part.
You can see full comparision between all editions of FMS here: http://www.adobe.com/products/flash-media-server-family/buying-guide-comparison.html