Kayaker,
The advantage of using the object type "Beams" for the steel connectors is you can output all your steel shapes for the whole project very quickly. If you design K-shapes, W-Shapes, or beam hangers with three different planes and do not unite them but instead name them with the correct "Designation Type" it makes it very easy to output the weight, volume, surface area, etc. In the attached zipped file you can see all the settings I have used to accomplish this.
Bringing the view of a plate into 2-D and determining the surface area and multiplying the thickness also works but can be a little slow for a large project. It also gets more complicated when you have a beam hanger with three different planes and then having to save each view.
As Jens said the middle mouse "Determine Values" is much easier. This way would be best for quickly determining the exact weight and volume of single pieces. As this is a fairly new function the values are only available in metric at this time.
When you output steel shapes designed as beams and not unite them, the weight and volume will sometimes be slightly higher as the program sees each piece as a rectangle to its longest point and will not take into account the small pieces that may have been cut away as seen in the attached picture. In most cases this can be an advantage and make for a more accurate quote as the fabricator will also be charging to the longest length of each piece.