K3B is part of KDE and Rhythmbox is part of GNOME. They are written in
different languages. The codecs would be the same, but not much else. K3B is
the best burning app for Linux, IMO. I do not rip CDs, but there are several
rippers in the repos, Ripoff, Ripper X, Asunder, Morituri, CDParanoia,
RubyRipper and Soundjuicer, plus several more console based apps. There is
also Grip: http://nostatic.org/grip/grip-download.shtml I think that Amarok
and VLC can also rip CDs although it will not be as straight forward as a
dedicated app.

You will need some restricted packages like LAME to rip using any of the
above.