However, for someone who knows neither, here are some reasons you might want to choose sed over Perl:
1. sed syntax is pervasive in other tools. For example, to run a substitution from early on in the tutorial in vim, type :%s/abc/(&)/<enter> from insert mode.
2. sed is simpler than Perl. It used to be that Perl filled a unique role as a scripting language but now there are a bunch of languages in that space (most notably Python and Ruby). Python + sed for example would fulfill most of the same functions that Perl does (and there are reasons to choose Python over Perl as scripting languages, although that's a much more complicated domain to discuss).
3. sed is more performant (or so I hear). This has never been a real concern for me, but some people cite this is a concern.
4. sed usually is a bit more terse. For the length of expressions you'll typically be writing with sed, this isn't a big concern.
Disclaimer: There are probably good reasons to choose Perl over sed, too. Not being a Perl guy, I don't know those reasons. I'll leave that to someone who knows more about Perl.
When I learned sed it was because I was taking a *nix class in college, and a professor pointed me at sed and not Perl. I learned sed instead of Perl because it was put in front of me, not because of any weighing of pros and cons. That's how a lot of learning happens. Sometimes simply learning what's put in front of you leads turns out to be an obvious mistake in retrospect (I was stuck writing VBA for a little while). I don't know whether learning Perl or sed is better for what sed does, but I do know that after maybe 6ish years using sed quite frequently it hasn't turned out to be an obvious mistake.