I'll start by discussing the one by Rabbi Angel first.
Physically, this is a smaller volume, quite comfortable in a tallit bag, for example. The translation is Rabbi Sacks's, from the Koren Siddur. Rabbi Angel's commentaries appear at the bottom of the page; they are a mix of historical context, references to other rabbinic texts, and associations with contemporary sources. For example, right there on mishna 1:1 he brings in a discussion of Daniel Kahneman's Thinking, Fast and Slow.
I particularly appreciated this third category. Avot suffers from overfamiliarity; Rabbi Angel makes it fresh. I was frustrated, in many cases, that due to the limits imposed by the size of the book and by the fact that it's aimed at the "one chapter a week" schedule, these newer ideas have to be limited to a few paragraphs. I would have loved to see them expanded into more complex and thorough explorations of the ideas, even at the expense of turning this into a volume that rewards slower progress through the tractate.
I feel this complaint even more when it comes to Rabbi Greenberg's edition.
Rabbi Greenberg's introduction is simply amazing and makes an audacious promise for the book. He starts by making two essential assertions, and supporting each of them with a carefully constructed argument.
First, that at various points in human history, God has chosen to become less manifest and more immanent. For example, the destruction of Jerusalem forced the replacement of the sacrificial service, uniquely located amidst pageantry and performed by a select few on behalf of all, with personal prayer in the home or local synagogue.
Second, that the transition from biblical to rabbinic Judaism, which resulted among other things in Pirke Avot, is an example of the how to both maintain the covenant and adapt to new cultural milieus.
The audacity comes in his assertion that we today are at another such turning point in Jewish history. The publication of this volume is the continuation of the essential work that Maggid and Koren are doing to provide Modern Orthodox editions of our critical texts and provide a welcome alternative to the hegemony of other perspectives.
And so I was disappointed that after the first few mishnayot, Rabbi Greenberg's commentaries became rather terse. In some cases, they seem to amount to an outline of what would have been an interesting dvar Torah, but as presented here too often they amount to a restatement of the mishna as bullet points, with one or two additional words indicating a particular perspective that Rabbi Greenberg brings to the text.
My recommendation? Well, I'm torn. Rabbi Greenberg's introduction to Pirke Avot is brilliant, and on some mishnayot his essay lives up to that promise. (3:15, for example, on the free-will paradox, is great.) Rabbi Angel's commentary is more consistent in depth -- not as thorough as Rabbi Greenberg's at his best, but with something meaningful to add to every mishna in the tractate. Both leave me grateful for what is there, but wanting to ask more questions, to read more depth. If I had to choose only one, it would be Rabbi Angel's. But if you can afford both, each offers a needed fresh perspective on this most essential text of the Torah Sheb'al Peh, the Oral Law.
(Full disclosure: As is often the case when I post a review of books from Koren, the publisher has provided me with complimentary review copies.)