The difference is that meters_per_second is an object,
not a type:

static const si::velocity meters_per_second;

I see.

At this point, I prefer to use the type, because
I don't like having to come up with two different
names for the type and the global constant.

That is a pain.

I suppose that it would also be possible to use only the
object, and use decltype whenever you need a type.

I tried using the decltype to get the type but got an error. Is this how you meant to use it?

typedef metric::nautical_mile_base_unit::unit_type nautical_mile_unit;

static const nautical_mile_unit nautical_mile;

quantity<decltype(nautical_mile)>(2.3 * nautical_mile);

Ryan