I'm a korn-shell veteran, so know that I speak from that perspective.
However, I have been comfortable with Bourne shell, ksh88, and ksh93, and for the most I know which features are supported in which. (I should skip ksh88 here, as it's not widely distributed anymore.)
For interactive use, take whatever fits your need. Experiment. I like being able to use the same shell for interactive use and for programming.
I went from ksh88 on SVR2 to tcsh, to ksh88sun (which added significant internationalisation support) and ksh93. I tried bash, and hated it because
it flattened my history. Then I discovered shopt -s lithist and all was well.
(The lithist option assures that newlines are preserved in your command
history.)
For shell programming, I'd seriously recommend ksh93 if you want a consistent programming language, good POSIX conformance, and good performance, as many common unix commands can be available as builtin functions.
If you want portability use at least both. And make sure you have a good test suite.
There are many subtle differences between shells. Consider for example reading from a pipe:
b=42 && echo one two three four |
read a b junk && echo $b
This will produce different results in different shells. The korn-shell runs pipelines from back to front; the last element in the pipeline runs in the current process. Bash did not support this useful behaviour until v4.x, and even then, it's not the default.
Another example illustrating consistency: The echo command itself, which was made obsolete by the split between BSD and SYSV unix, and each introduced their own convention for not printing newlines (and other behaviour). The result of this can still be seen in many 'configure' scripts.
Ksh took a radical approach to that - and introduced the print command, which actually supports both methods (the -n option from BSD, and the trailing \c special character from SYSV)
However, for serious systems programming I'd recommend something other than a shell, like python, perl. Or take it a step further, and use a platform like puppet - which allows you to watch and correct the state of whole clusters of systems, with good auditing.
Shell programming is like swimming in uncharted waters, or worse.
Programming in any language requires familiarity with its syntax, its interfaces and behaviour. Shell programming isn't any different.
