RE: Finishing for a TF GS P-51D
Use both.
Use gloss anywhere you are placing a water slide decal. This eliminates the silvering edges you will see on models that have had them applied over flat surfaces. Spot paint a smooth clear spot, apply the decal, then once all of them are applied shoot the semi flat clear coat to seal it all up. Even the dry transfers can benfit from this but sometimes I have found they dont stick to high gloss surfaces so testing is required to prove this. Dry transfers can be funny based on how old they are, the color, and the base material.
After a battle with dry transfers not working my buddy and I switched gears and laser cut stencils to paint all of the maintainer call outs on a PST F-84. This ultimately looked much better than anything else.