I had the same question here and it came down to this…
It depends! (yep that’s it). This is what Otto said…
Your question cannot be answered because you don’t give a proper
scenario or framework by which to answer it.Searching by meta value only is inefficient because it is not an
indexed value. Searching by key and value is not as bad, because the
key is indexed. Still bad, but not as bad for small sets. For it to be
noticably bad, you would need to have multiple tens of thousands of
posts and millions of meta. A large news site, for example.If you’re talking about a rather small blog, then do what you like.
Everything depends on scale.No quantitative data exists because the answer is self-evident to
anybody who understands databases and basic SQL queries. There is no
need to measure that which is obvious.
If you’re talking about many 1000s of posts then taxonomies might be better.