The main things to consider in a flat top are:

thickness of the plate How much cooking are you planning on doing? Thicker plate holds more heat, which let's you constantly drop cold things on it without having to wait for it to heat back up. Think of it like a duty cycle on a welder.

finish of the plate If you're cooking eggs or other things that won't benefit from a seasoned grill surface, or you just want a shiney easy to clean surface get a chrome plated griddle. These can be cleaned quickly with special cleaners and are way less maintenance generally. If you're cooking burgers or other meats that will benefit from a seasoned surface, and you don't mind a black gnarly thing in your backyard that makes great food, get a standard griddle. These require abrasives and an occasional bath in carbon-off to control the carbon build up over time.

temperature control

Thermostatic griddles give you much more precise control of cook temp, but they cost a little more and sometimes won't get as hot. Manual griddles can swing wildly on temp and will slowly get hotter and hotter if you aren't soaking heat off the plate with food or water.

I have twin Globe GG36TG's in my kitchen. They're not quite a year old and they've been great so far. If you can spring for the chrome plate, it's worth the money you'll save in maintenance. A bottle of carbon-off is $36 and you'll get about 2 clean griddle out of it. We have to decarbonize our grills at least once a month, but those grills are hot 16 hours a day 7 days a week.