I have tried both methods and here are my thoughts. Try building and painting ins "sub assemblys". They may not necessarily correspond to what the instructions say. I find it easier to paint as much of the major portions of the project assembled together as I can. On tracked vehicles, paint the tracks first and if they are indy tracks, paint them first on the sprues and then touch them up after. For rubber ban tracks, paint them before you attach the ends. Try pinning them in a cut out hole in posterboard. Cut a hole just slightly shorter and higher than the track and pin the ends to the foam core board. Then you can paint both sides at the same time. Most new Tamiya tracks let you use regular model glue on them but I still use super glue.
Paint the tools after they are attached to the vehicle and after they have received a base coat of your vehicle colour. You will cover up any areas where glue can be seen on the vehicle. Also, a lot of the new glues do not stick worth a darn on painted surfaces. It is not that terribly hard to paint the equipment on the vehicle and after you have done a few it goes pretty smoothly.
If you are doing something like a Marder or a Wespe with an open crew area, you may have to alter the building sequence a bit so you can paint the crew compartment before you attach the main gun. Either that or make sure and paint very thoroughly with your airbrush from many different angle.
All of the above applies to armour and for aircraft or boats and whatever....................switch to armour.