Everyone's responses are right, its statistically better to put it in all at once because the market averages 8-10% up per year and you run the risk of missing on the next 6-12 months of gains.
However, if that $150,000 is a significant portion of your net worth, its probably better to do the lower risk/lower return method of dollar cost averaging over 6-18 months.
There's also a third strategy....called "When the bear shows up, throw money at it until it goes away". Basically invest in chunks on the days the news says "Down down 200+ points! The world is ending"