Flats or gloss?

it really is an it depends.  If you look at a Saturn 1, 1B, or V you can use the gloss BUT when you see it on the pad fueled, you would use the flat because the white coating is ice (watch the Saturn launch videos and watch the falling ice).  As far as the shuttle, take a look at the ceramic tiles, they are not glossy.  The Apollo is foil covered once the launch shroud is off but with the shoud on, it can be glossy.  The LM is definately not glossy on the black parts and the silver colored parts are more similiar to the reverse side of aluminum foil.

Bottom line is this: at what phase of a mission do you want to replicate the vehicle.  Another option is if you want to replicate the handmade models presented to VIPs in the program.  If you want to do the latter, gloss is the answer.  The final and most important answer, is which one do you personally like the best?   I personally use a combination of all the above depending on where on the vehicle the part is; is it prelaunch or post missionl and whether I am keeping it for myself or giving it as a gift.  The gloss is much easier for people to maintain than the flats.

Hope this helps. 

You should also take the scale factor in consideration.  When you look at objects from great distances, they look more "flat" than "glossy" simply because you can't see the reflection of nearby objects on their paint. 

If you make a 1/96 Saturn V with glossy paint, you might end up seeing the surrounding object reflecting on it, which is not very realistic. 

Here is my own personal rules:

If the thing I want to model is flat or semi-gloss in reality like armor or war aircraft, I will spray a dead flat coat on my model.

If the thing I want to model is glossy in reality like spacecrafts, I will spray a semi-gloss coat on it.

But, for cars, if it's a big scale like 1/24 or bigger, I will use glossy finish.  For smaller scales, I will use semi-gloss finish.

You can achieve the finish you want by using Future floor polish mixed with Tamiya flat base at different ratios. 

I recently completed the Revell ISS and I can tell you that gloss did NOT work!

It did not look realistic so I did the model in flats. There are photos on here somewhere however they don't look as good as looking at the model in real life. I'm not much of a model photographer.

The Tamiya gloss clear blue I used on the arrays looked like candy (and I'm a pretty good painter). So I stripped the ones that I did and did them in MM flat sea blue , and Tamiya gold leaf thinned (as to remove the gloss).

Just my thoughts on this.