http://arduino.cc/en/Reference/Float:
Datatype for floating-point numbers, a number that has a decimal point. Floating-point numbers are often used to approximate analog and continuous values because they have greater resolution than integers. Floating-point numbers can be as large as 3.4028235E+38 and as low as -3.4028235E+38. They are stored as 32 bits (4 bytes) of information.
Floats have only 6-7 decimal digits of precision. That means the total number of digits, not the number to the right of the decimal point.
http://arduino.cc/en/Reference/Long:
Long variables are extended size variables for number storage, and store 32 bits (4 bytes), from -2,147,483,648 to 2,147,483,647.
Bei long hast Du bis zu 10 significante Stellen; bei float kannst Du grüßere Zahlen darstellen aber nur mit 6-7 significante Stellen.
Was ist genauer?
Grüße Uwe