Common ground: expenditure from real data
Formulas like Mifflin-St Jeor estimate your calorie needs from height, weight, age and activity. That is a starting value, nothing more. Both apps go a step further: they compare what you eat with what your weight actually does and derive your real expenditure from it. If you log cleanly for two or three weeks, you get a much more reliable value than from any calculator.
Difference 1: where the data comes from
MacroFactor wants you to eat and log inside the app. Verified food database, barcode scanner, custom foods and recipes, micronutrients: food logging is the core, and it is mature.
Lean is law has no food database. You enter calories and optionally protein, carbs and fat, use recently used entries or read the values from Apple Health or Health Connect. That is faster if you eat set meals anyway or already use another app, but less convenient if you want to log every ingredient individually.
Difference 2: goal or phase
MacroFactor works with a goal (lose, maintain, gain) and adjusts your calories and macros in weekly check-ins.
Lean is law works with phases, the way bodybuilders plan: Bulk, Cut, Maintain and Diet Break. Each phase has a rate in percent of your trend weight per week (Bulk default 0.35 %, Cut 0.7 %), you can plan the next phase in advance, and the app warns when the rate is too fast. Maintain and Diet Break run with a corridor that, in Pro, also warns early if the trend is about to leave it.
Difference 3: what happens besides eating
Lean is law bundles what bodybuilders otherwise keep in several apps or notes: weekly check-in with posing photos, supplement list with reminders and ticking off, experiments and markers (such as a refeed or travel) with before-and-after evaluation in Pro. According to its store description, MacroFactor offers habit and cycle trackers, among other things, and its own app for training.
Difference 4: price and language
MacroFactor is subscription only after a 7-day trial, and the app is in English and Japanese. Lean is law starts free with no time limit, Pro is available monthly, yearly or as a one-time purchase, and the app speaks ten languages.
Verdict
If you want to track your nutrition down to the detail, MacroFactor is the more complete tool. If you mainly want to know whether your bulk or cut is running at the right rate, and need trend, phases and check-ins in one place for that, Lean is law is the leaner and cheaper choice.