assembly/paint question

If it can be attached cleanly (unlikely to result in glue mishaps and not requiring any sanding/filling) AND if it will be difficult to mask off after attaching (because it's to have a different color than the part it's attached to), paint the part in question before attaching.  Otherwise, attach first.

As I ended up finding out the hard way, each model and each subassembly will have a different strategy for how to attack assembly and painting, and you always want to consider what other parts may be going together with that subassembly in later steps.  It never hurts to jump a few steps ahead in the instructions and do a test fit before committing to a plan for what you're currently working on.  Its really a bummer to assemble and paint something in a way you thought made sense, only to find out that the manufacturer had a different idea when you try to attach that subassembly to the rest of the model.

One of those questions you need to address before starting any work.  For most of us it is build-a-little, paint a little, repeat.  Looking at the instructions carefully, you can plan the sequence.  On some of the top quality kits, they show paint numbers in those steps where you need to paint parts.  No law says you have to follow that, but if you do not have those parts painted by completion of the step, you may have problems doing them later.  Good example is cockpits.  If you do not have cockpit parts before inserting the cockpit into fuselage, it may be hard to paint parts later.

It's almost always a combintion of all of the above. Some items you paint as you go and mask off, others get painted after assembly, and then details are painted afterwards.

Past is experience, present is experiment, future is expectation

I'm at the second point 

When I do armor I usually assemble almost everything, just leaving off the roadwheels,tracks, antennas,stowage,and figures,and airbrush they entire thing.If tools  need painting,I slide a post it under or next to it as a mask and carefully paint by hand.

I don't really see it as a dilemma,just a process,which varies from cars,armor,ships and planes,I just plan ahead the best process in my head.