Right, in this case a custom PC is a good idea. A good non-mobile CPU will be great for 3D and video. You'll have the option to use proper expansion cards for audio stuff too.

Other than paying more, when going the laptop + dock route you lose some performance. For most uses it doesn't matter (games, internet), but for content creation you want as much power as possible. The positives are much lower power consumption, a free UPS (laptop's battery), and mobility (just unplug the dock and work on the go).