Some people prefer to invest using a monthly savings plan in order to reduce the chance of them putting their money into the market at a “bad” time. With monthly payments you can benefit from what is called “cost-dollar-averaging”.
However, statistically, it makes sense to invest the full amount all at once. This method maximises the time your money is invested for. Many studies show that the amount of time on the market is one of the most important factors in successful stock market investing.
You can of course use a combination of both a lump sum initial investment and a savings plan.