My answer would be 'pick one and learn how to use it'. They're both decent shells; bash probably has more bells and whistles, but they both have the basic features you'll want. bash is more universally available these days. If you're using Linux all the time, just stick with it.
If you're programming, trying to stick to plain 'sh' for portability is good practice, but then with bash available so widely these days that bit of advice is probably a bit old-fashioned.
Learn how to use completion and your shell history; read the manpage occasionally and try to learn a few new things.
