Funny you should ask. I have just taken delivery of an old Vespa and have laid down my thoughts at considerable length on my blog. If shifting is the sole consideration then take the GT200 based on my experiences with the GTS250. But either way good luck with the splitting exhaust pipe issue, required when taking the rear wheel off which is hard enough as it is and dealing with the belt. Honesly they make roadside maintenance a nerve wracking problem for me. Especially as tires wear out every three thousand miles and belts could last ten thousand miles. You'll be. Along way from home. I know. I've done it.
Take this is encouragement not to stop you doing what you want. Get reading and learn all you can.