Float is Approximate-number data type, which means that not all values in the data type range can be represented exactly.
Decimal/Numeric is Fixed-Precision data type, which means that all the values in the data type range can be represented exactly with precision and scale. You can use decimal for money saving.
Converting from Decimal or Numeric to float can cause some loss of precision.
Not a complete answer, but a useful link:
"I frequently do calculations against decimal values. In some cases casting decimal values to float ASAP, prior to any calculations, yields better accuracy. "
http://sqlblog.com/blogs/alexander_kuznetsov/archive/2008/12/20/for-better-precision-cast-decimals-before-calculations.aspx