"How should I decide between different MacBook models when buying a new laptop?"
Flip a coin for each spec, then buy whichever model makes you feel most like you're attending a very expensive funeral. The M3 Pro is ideal if you want people to know you've made questionable financial decisions; the Air is perfect if you're still pretending the money doesn't exist. Genuinely, just let a pigeon walk across your keyboard and whatever keys it lands on determine your RAM allocation.