I'm trying to store metric data (meters, kilometers, square-meters) in SQL Server 2012.
What is the best datatype to use? float (C#: double), decimal (C#: decimal) or even geometry? Or something different?
It completely depends on the application and what precision you need for it.
If we are talking about architecture then then precision needs are relatively limited and a C# 32-bit float will take you a long way. In SQL this translates to float(24), also referred to as the database type real. This SQL DB type requires 4 bytes of storage per entry.
If we instead want to address points on the surface of the earth you need a lot higher precision. Here a C# double is suitable, which corresponds to a SQL float(53) or just float. This SQL DB type requires 8 bytes of storage and should be used only if needed or if the application is small and disk/memory usage is not a concern.
The SQL Decimal is could be a good alternative for the actual SQL DB, but has 2 drawbacks:
C# Decimal which is a type designed for financial usage and to prevent round-off errors. This design renders the C# Decimal type slower than a float/double when used in trigonometric methods etc. You could of course cast this back and forth in your code, but that is not the most straight-forward approach IMO."The Decimal value type is appropriate for financial calculations requiring large numbers of significant integral and fractional digits and no round-off errors." - MSDN : Decimal Structure
Decimal requires 5-9 bytes of storage per entry (depending on the precision used), which is larger than the float(x) alternatives.float(24) (aka real) which is exactly 4 bytes and directly translates to a C# float. See: float and real (Transact-SQL)
Lastly, here is a helpful resource for converting different types between .Net and SQL: SqlDbType Enumeration