are the main frats/sororities really that different??

Obviously, you won't have the exact same experience in every frat or sorority. You will meet different people. But overall, yes you do the same things. All of the top 5 frats/sororitys have sisterhood/brotherhood events, compete in charity competitions, formal, ritual, recruitment, rush, parties, weekly socials etc. you just do it with different people. Some organizations emphasize different things slightly more but they all do everything.

If you're looking to join a frat/sorority, choose it based on who you connect with the most because structurally, they're all very similar. Can't say the same for the lesser known frats because they don't do nearly as many weekly social events.