Personally, I really like Sunday for finishing up shopping and hanging out with friends before we head home.
Artists in the Alley often like the discount their stuff a little on Sunday (since they often don't want to have to pack it all back up to bring home), so that's always an added plus for me. I sometimes see the merchants in the Dealer's Hall do this as well, so if shopping is your game, Sunday is a great time to do it.
Other than that, Sunday is a great chance to get any last-minute photos you might've missed out on earlier in the weekend. I like to grab food with a huge group of friends too, and talk about anything that happened while we split up (ex, if we went to different photoshoots or panels, etc). Sometimes we see some more programming too, but only if something really sticks out to us as interesting.
In your case, I would definitely check out the programming schedule, and see if anything really strikes your fancy. If you know you're bringing a bunch of cosplays, only going two days might not give you enough time to go out and enjoy wearing them. If you have another con coming up soon, though, and want to save the money for that, it might indeed not be worth staying.
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