I'd go for the GT200. I own one, bought it back in 2016 with 1800 miles on it (2006 model) in pretty much brand new condition. I absolutely love it. It now has just over 14,000 miles on it, and the only problems I have had with it, right after buying it, and probably caused by it sitting for so long, were a failed vacuum petcock and a leaking water pump. Other than those, I have not had a single issue with it. But if I need to, I can work on a carbureted bike. Been doing it for decades. They are several times simpler than computerized EFI, and don't have any failure prone electronics. People have put over 70,000 miles on the original engine. When something goes wrong with an EFI bike, and it will, you pretty much have to take it to a dealer, and it will be very expensive. Most people don't realize just how complex EFI is. It's not just replacing a carburetor with an electronic throttle body. There is a high pressure electric fuel pump, at least one ECU (computer) at least 7 sensors that send information to the computer, and connections from the computer to the throttle body. In all newer EFI bikes, the ignition is also controlled by the EFI. EFI has become an electronic vehicle management system, controlling everything.
Unless it already has problems, I wouldn't worry about parts for a GT200. There is a LOT less to go wrong with a GT200 than a GTS300. And at 10 years old, it won't be long till parts start becoming hard to find for the GTS300, now that the old GTS300 has been replaced by the HPE. I did the 12,000 mile service on my GT200, and all the parts were available from Scooterwest. There are also a number of aftermarket parts available for it.
2006 Vespa GT200