A flat top does things grates cannot (eggs, smash burgers, onions, anything small enough to fall through) and asks for one thing in return: it must be seasoned and kept dry, or it rusts.
A griddle surface is bare carbon steel protected only by a baked-on layer of polymerised oil. That layer is what stops rust and makes the surface non-stick, and it builds up over time with use. Strip it with harsh detergent or leave the griddle wet and you start again. A cover is not optional equipment on a griddle.
Griddles produce far more runoff than grates, and where it goes is a real design difference. Rear-drain and front-drain layouts each have advocates; what matters is that the channel actually carries grease to a catch cup you can reach and empty, rather than pooling at one corner.
The point of a large griddle is cooking several things at different temperatures at once: eggs gently at one end, a hard sear at the other. That only works if the burners are separately controlled, so count the independently adjustable zones rather than the total burner count.
A thicker cooktop holds heat through a batch of cold food and recovers faster between rounds, which is exactly what you want for smash burgers. Thinner plates heat up quicker and develop hot spots. Thickness is rarely advertised prominently and matters more than the BTU figure.
If the griddle lives outdoors uncovered, plan on re-seasoning it seasonally; if it lives under a cover, seasoning becomes an occasional touch-up rather than a chore.
More depth: gas grills picked by budget and when grills actually go on sale.
Wash off the shipping coating, dry it completely, then heat the plate and wipe on a very thin layer of high smoke point oil, letting each layer smoke off before adding the next. Several thin layers build a hard, dark, non-stick surface. Thick layers stay tacky and peel, so thin is the important part.
For some cooking, comfortably: it is better than grates for breakfast, smash burgers, fried rice and anything small. It cannot give you grill marks, the char that comes from flame contact, or true indirect cooking for large cuts, so many people keep both rather than swapping.
Only if the seasoning is damaged or the plate is stored wet. Bare carbon steel rusts readily, which is why the baked-on oil layer matters and why a cover is essential outdoors. Surface rust can be scoured off and the plate re-seasoned, so it is recoverable rather than fatal.