I’ve used them commercially - don’t worry I’m not about to suggest something that costs £6k or anything.
Biggest thing is to look at how thick the hot plate is - the thicker it is, the less the temperature will drop when you put food on it. Not such an issue for a fried egg maybe, but more significant if you’re using it for steak / burgers etc.
Something that might be more of a subjective thing, but I’d never buy any griddle with the ridges in it. You’ll only get the browning happening along the ridges, whereas really you want it over the whole surface of a piece of meat.
Keeping cast iron flat-tops well maintained is a lot easier than doing a full scrub down of a stainless steel one imho - but you do need to ensure it remains properly dry. Stainless will also stick more at higher temperatures (generally the temperatures you want for searing steaks and burgers), whereas cast iron doesn’t once it’s well seasoned. If it’s a frying pan you’re using stainless isn’t such an issue because you can then deglaze the pan and have a lovely sauce, but that’s tricky on a flat top.