As a response to the reviewer who pointed out that this costs more money than "Anthology", but contains many fewer songs, you're correct.
However, "Hits" (as well as "Misses") transcribes each song using the tuning Joni used to play it. Joni is notorious for using alternative tunings on all her songs (except for the piano based ones of course). She's used about 50 different tunings (!), and never uses standard tuning on her songs.
And that's the key - "Joni Mitchell Anthology" assumes everything in standing tuning (EADGBE). This makes things complicated; for example, if you're playing "Chelsea Morning" in standard tuning versus the tuning Joni played it in (open E tuning I think), it's totally different. The voicings won't be the same, and it'll probably be difficult, because the chords are strange. However, the "Hits" book tells you the correct tuning. And turns out, it's a very easy song to play. And it sounds perfect. Joel Bernstein did a fantastic job. He also knows Joni, so he's not just guessing with these tunings - they're the real deal.
So yes, you're not getting as many songs with "Hits". But it's a small price to pay for true accuracy. So it comes down to this:
If you want to try and play Joni's songs just like she does, "Hits" (and "Misses") is the book for you.
If you don't want to fumble around with weird tunings, and want a large library of songs, stick with "Anthology".