Two lids fit the same cup. They cost the same. One of them will cost you remakes and the other will not, and which is which depends entirely on what a finished serving looks like when it leaves your counter.
Most shops make this decision once, by accident, and then live with it for years.
The question is not the lid, it is the profile of the serving
Look at a finished cup from the side. If the dessert sits level with the rim, a flat lid is right. If it mounds above the rim, because of a tall scoop, a whipped topping or a layered build, you need the clearance of a dome or you will be pressing a lid down through the product.
That is the whole test. Everything below is a consequence of it.
What a dome buys you
Dome lids for 6 and 8 oz cups carry a raised profile that leaves headroom above the rim, so tall scoops and topping builds survive the trip instead of arriving flattened.
The clear plastic matters more than it sounds. A clear dome keeps the dessert visible, which is worth something in a display case and worth considerably more in a delivery bag, where the lid is the first thing a customer sees rather than the dessert.
What it costs you is space. Domed cups do not stack. A shelf of them holds far fewer than a shelf of flat-lidded cups, and in a small back freezer that is a genuine constraint rather than a detail.
Self-serve is a special case
If customers build their own portions, you cannot predict height. Some fill level, some pile. A dome is the safe default because it accommodates both, which is the reason a 16 oz dome exists for frozen yogurt counters where portion height varies by customer rather than by recipe.
What a flat lid buys you
Flat lids let filled cups stack. If you pre-portion ahead of a rush, hold prepared servings in the freezer, or lay cups out for an event, that stacking is the difference between fitting a service into the space you have and not fitting it.
Catering is where the gap shows most clearly. A hundred pre-portioned cups on flat lids take up a fraction of the footprint that the same hundred take up under domes.
The trade is plain: nothing can sit above the rim. Fill level, lid on, done. A shop that runs deliberately generous scoops will find flat lids arguing with it every single service.
Running both, which is where most shops land
No rule says pick one. The common arrangement is flat lids for anything pre-portioned or held, and dome lids for anything built to order in front of the customer. Two lines on the shelf, each with an obvious purpose, and no one standing there deciding.
Sizing makes that easier than it sounds. The 6 and 8 oz share a lid in both profiles, so covering two cup sizes takes two lid lines rather than four. That shared sizing is one of the quieter arguments for the 8 oz being a standard, which we got into when choosing an 8 oz cup. Larger cups carry their own, and the full set sits in the lids and domes range.
One thing that is not about profile at all
Some to-go lids are made without vent holes. An unvented lid keeps the seal closed in transit, which matters when a frozen dessert travels in a delivery bag alongside warm orders. If most of your volume leaves the building rather than crossing the counter, that is worth checking before profile, before color, before anything.