What is the most dependable (well built) 150cc scooter on the market, and is a "trike" actually safer?

Vespa is the best built bike on the market and it is pricy. With all bikes, you lean into the turn. The trike is kinda the same. With a trike, you make wider turns. Realize your limitations. In a turn, one of the back wheels will leave the ground and then it is sheer balance. You can make a much sharper turn on two wheels. In a lot of ways, the trike is more stable.

You dont say what country you are from but assuming you are UK quad bikes are now legal on the road and will make you feel safer if thats what you want. A trike or a quad will enable you to turn the bike by turning the front wheel where as a two wheeled bike you do have to lean it to turn it. I would recommend having at least a 100cc or larger to enable you to keep up with the traffic which means you should be safer as it means some dumb people wont be overtaking you dangerously (I know what its like).

There are convention trikes 1 at the front 2 at the back there's also one which is 2 at the front and one at the back, check it out could be what your looking for.