Moldvay vs Mentzer

What's striking to me is how much is the same between both. Even short, throwaway rule guidelines from the Moldvay book are carried faithfully forward by Mentzer into his revision. It's clear to me that Mentzer did not intend to change the rules much at all, merely re-edit and present them in a more expansive, instructional format. The few changes he did make are mostly to be more generous to players, for example encumbrance limits are raised. The big exception are the Thief skills, which were a casualty of marketing - they had to be stretched out to 36 levels, so their actual gain per level is slower, which makes the Thief more useless for longer.  The actual mechanics, however, such as movement, speed, combat, item prices, etc. are exactly the same.

Neither Basic set contains a method for random dungeon generation. Instead they contain guidelines on building a dungeon and how to stock it, but the actual layout and method is left up to the DM.  The Mentzer Basic book is actually the same as the earlier revision's, just expanded. This makes the AD&D random dungeon generation table (copied from an article from TSR magazine) unique.

The big pillars of the OSR community are the Moldvay, Cook and Marsh B/X sets, and Allston's Rule Cyclopedia. The BECMI sets sort of get lost between them, since they're assumed to be superseded by the Rules Cyclopedia. The value of B/X and Rule Cyclopedia is that they're both written as reference works, and thus much easier to look up during actual play, while the big flaw of the BECMI sets is that the rules are divided between Players' books and DM's books, and the rules, items, and magic lists are scattered between the 5 sets, making them tedious to look through during a campaign.

Fans of B/X prefer its "simplicity". Restricting itself to 14 levels (though it Cook's Expert hints at 36), the Basic and Expert sets cover dungeon crawling and overland adventures, which is the same focus as D&D's original Underworld and Wilderness Adventures, and to it's proponents that's all you need. It's hard to argue otherwise, since modern D&D doesn't do anything different.

The Rules Cyclopedia, on the other hand, combines everything from the development of the Basic/Classic line into one book. This means it pulls in optional material such as skills and demihuman classes from the Gazetteer books, the large scale combat and dominion rules from the Companion set, and the weapon mastery and immortal paths from the Master sets, and even some hints from AD&D 2e. It takes an "everything but the kitchen sink" approach to the rules and those who have it give it nothing but praise for combining everything about D&D you would ever need into one volume.

However, people who quibble about the rules differences between the editions are barking up the wrong tree.  The Rules Cyclopedia replaces some of the edits put in by Mentzer by carrying forward the original rules written by Moldvay, but then in other places uses the same text as Mentzer's BECMI set. The Classic D&D line does not have the major differences between editions that AD&D 1e and 2e, or D&D 3, 4, and 5e have had.