Can you really judge a book by its cover? To some extent, this is a good starting point – each commentary series has its own goals, and relative technical difficulty level. Best Commentaries dot com takes you further, though – you can see the books on a virtual bookshelf (comparing their relative sizes and thicknesses), and click on them to see their covers. Additionally, they’re all scored, based on an aggregate of different reviews. You can also see a list of forthcoming commentaries on a book. Well worth a look, if only to get a feel for how many commentaries are out there.