TeslaWRLD,
I know you are concerned about potential service costs. I would think there would a larger concern for what you are going to use the different scoots for. There really is no comparison. The 150 is a more of an in town scoot. The GTS300 has the get up and go for freeways and long distances.
Even though I always wanted the classic Vespa, the 150 was just not enough for me. I didn't even buy 250 when it came out. When the GTS300 came out, I bought it.
Service costs and intervals were not as important because I used my scooters mostly for recreation or occasionally running to the market. I was putting on less then 2000 miles a year. Further, the riding Season here is less than 7 months.
Good luck on your choice.
Bob Copeland