I do a lot of aircraft and space vessels in C4. I don't yet have C5, but I also have most of the other 3D apps like Lightwave, Maya, 3Ds Max, etc. I tend to use Carrara most often due to it's simple interface and the way you can just scroll around with less hassle. It really depends on what you are doing however. I sometimes pull at my hair in frustration when trying to design a particular piece for a hull or fuselage though. It would be nice if you could map points in the main assembly room, of two objects and line everything up in there. The basic thing you have to be able to do is to visualize the part and how it is composed. Like an engine cowling for a radial engine would be best done as an extruded shape using the front plane as the drawing plane. For wings I usually use a series of ribs with 4 vertexes each as looked at on the side. As for which is best, each package has their own good and bad points but I usually recommend Carrara because it is easy and fast but has some very powerful capabilities for any level of proficiency.