Also the rectangle created by createShape seems have a different strokeWeight and smooth value than the one created by rect(), anybody knows why is that?

rect() and related methods are direct drawing methods. They have been around for a long while in Processing and they directly draw to the screen whatever you tell them using the latest fill() and stroke() settings. They are affected by rectMode, ellipseMode, etc. For custom shapes you can use beginShape-endShape.

With Processing 2.0's new OpenGL renderers comes the possibility to create retained PShapes using the createShape() method. These are shapes that are created ONCE, stored and therefore can be reused with maximum efficiency. The reason to do this is to improve performance. To compare, it's stored PShape objects versus direct drawing shape methods. So these PShape objects are not affected by the current fill() and stroke() settings. Instead they have individual stroke() and fill() settings.

The createShape name parameters like RECT etc. are just convenience methods that quickly draw for you a beginShape()-endShape(). If these default convenience methods don't give the desired result, it is very easy to just draw whatever shape you want yourself. In this case by supplying 4 vertices to make a rectangle positioned exactly how you want it in relation to the shape's center.

For more info, see: http://processing.org/tutorials/pshape/