So, for those of you who have experience, what separates BECMI from AD&D, both in rules and flavor?
BECMI is more straightforward and less fiddly than AD&D. Your OSE will have the same basic base as BECMI.
3d6 in order for generating stats is the default with some options for trading around some stat points.
Stats all have modifiers going to +3 and use the same modifier chart.
No percentile strength.
All classes are slightly lower powered than AD&D. Fighters have d8 HD, clerics have d6, thieves have d4. Clerics don't get spells at first level or bonus spells based on wisdom.
Race as class. All elves are warrior wizards. All dwarves are fighters. All halflings are sneaky fighters.
Race gives no adjustment for stats.
Some neat things in the classes. Dwarves can search for traps decently.
Weapons do more straightforward damage d4, d6, d8, d10 or all weapons do a d6 (DM options).
No weapon proficiency, but with the master set and RC you can have weapon mastery specializations.
Spells have a range and duration, no components or spell schools or other details.
Initiative is different, side based and phase of combat based (movement, missile fire, melee, spells).
Morale for monsters and NPCs is a thing unlike in 1e AD&D.
Alignment is Law/Chaos/Neutrality. No Good/Evil axis.
More emphasis on tracking time in the dungeon in basic. Doing many activities takes a full turn, wandering monsters are checked on a basis where turns become relevant. Light sources last X turns. Etc.
Prices of gear are different, you can start as a 1st level fighter in platemail and have the best non-magical armor in the game.
Flavor having no half-races or gnomes or subraces is distinct.