I've searched a bit, and from what I can see, both Geometry and Gography stores the coordinates as IEEE 754 64-bit float internally, hence there is no difference in precision... Please review the following code:

DECLARE @geo geography;

SET @geo = geography::Point(1.234567890123456789, 1.234567890123456789, 4326)

SELECT @geo.ToString();

go

DECLARE @geo geometry;

SET @geo = geometry::Point(1.234567890123456789, 1.234567890123456789, 4326)

SELECT @geo.ToString();

go

Both select statements return the same:

POINT (1.2345678901234567 1.2345678901234567)