If you have not rode before, the 150 would definitely be a good choice.
Also, it would not mean you are locked in forever. I good measure of the folks
here on ModernVespa moved up to larger displacement scooters over time.
1. If you are not 100% sold on riding, the 150 is less of a financial commitment.
I say this because a good many are initially excited then don't stay involved and
the scoot sits in the garage unridden.
2. If you get crazy about riding, you can trade up at anytime.
On balance, I think the Vespa 300 is the right scooter for all purposes. It has the
classic look and power to get up and go on the freeway. You can also ride it
slow on side streets.
Think of it this way - the 300cc is really a small motorcycle. Not to big for you.
Go luck on your decision and the opportunity to have so much fun on two wheels.
Bob Copeland
Minnesota