How does jazz from the 30's and later compare to jazz today?  

Jazz then is not all that different from Jazz now. That music is still being performed by live bands, people still dance to it and just like then some bands and performing artists are better at it than others. However, it should be noted that America's early Jazz scene predates Ella Fitgerald and Duke Ellington.

I would say the only difference now is that some jazz of today is slightly more bluesy then way back..Other than that I really dont see how its changed much to be honest with you...