How do I decide between flat fee or per image pricing

Flat fee means predictable costs and full coverage
When you pay one price for the entire wedding, you get complete coverage and don't worry about overage charges. This works well if you know you want comprehensive documentation and prefer a set budget. The photographer has no incentive to deliver fewer images, and you aren't penalized for a longer event.
Per image pricing gives control but uncertainty
Some photographers charge per delivered image. This can be affordable if you want a smaller collection, but your final bill depends on how many keepers the photographer delivers. If you want only 200 edited images, this might save money. If you want 500, it becomes expensive. Confirm whether the photographer edits conservatively or generously.
Hybrid models exist
Other photographers charge a base fee plus per-image fees, or offer tiered packages: bronze, silver, gold with different hour counts and image quantities. Compare what you actually get in each package, not just the price. The cheapest isn't always the best value if coverage hours are too short for your event.