Q:

Tube vs. Cylinder

What is the practical difference between plotting Tubes and Cylinders?
The documentation for each describe a 3D tube/cylinder around the line between 2 points....they seem identical.
However, there is clearly a difference as I notice that 
C1 = Cylinder[{h1, hc}, .5]
RD1 = RegionDifference[T1, C1];

works great, but 
C1 = Tube[{h1, hc}, .5]
RD1 = RegionDifference[T1, C1];

spits back an error that Tube[...] is not a correctly specified region.
When should I use one or the other?

A:

There are lots of places where you can indeed replace Cylinder by Tube, but Tube allows more styling options and flexibility. Most of this information can be collected from the documentation, so I'll just list some aspects that I think are especially noteworthy:

In terms of regions, it seems to me that you already answered the question: you can't use Tube there, as the error message says. 
Tube supports VertexColors and Cylinder doesn't. 
Cylinder has a fixed radius, and Tube allows a variable radius so you can make non-cylindrical shapes with it.
You can also use Tube with a single argument as a PlotStyle option in ParametricPlot3D.
Tube can extend across more than two points, and even along a curve.

A:

You can't make worms with a cylinder ... but a Tube:
Graphics3D[
 Tube[BSplineCurve[{{1, 1, -1}, {2, 2, 1}, {3, 3, -1}, {3, 4, 1}}], .5]]

Tube takes a Line or curve to build itself around, or two endpoints. It also generates caps in a different way.

