There is a distinct difference in precision between the two.
Do you need precise values to 20+ decimal places? Are you dealing with fractions of a cent or calculating rocket trajectories? Would being off by .0000000000000000000000000000001 have you looking for missing hikers 100 miles away from their last known coordinates? Do lives hang in the balance? Use doubles.
Do you intend to round the value up or down, can't measure values to that degree or don't inherently care how many decimal places there are? Are you measuring the temperature outside? How long it took to run a quarter-mile? The sum of the change in your pocket? Use floats.