Gloss or flat clear coat?

Slightly different applications. For either, you want a clear gloss coat for the decals to minimize silvering. For planes, most people will do pin washes to flow into the recessed panel lines, and not have washes affect the overall color of the plane. Hence the gloss coat for planes, the wash wills simply not affect smooth areas or have a minimal effect for dirt.. For armor, modellers will do both pin wash, and much more drastic changes in color. A mattle coat (or satin if you want something in between) is used, so more of the base color is affected by the overall wash. Not to mention streaks, etc... You will find that if you used a glossed coat, it will be very difficult to create streaks as they are too easily blended away.

I use a gloss coat on my armour before weatehring and a lot of the armour builders i know do the same. But i guess it will depend what your doing. A lot of armour modelers will use filters of some sort and these are best applied to a gloss or semi gloss finish.

If i want a more grubby look to my aircrfat then i will do that over a flat finbish as it helps to grab more of the wash when i remove excess.