Hawk,
I definately prefer the first method - flat paint, Future, decal, dull coat..............
It's much easier to get opacity with an extremely thin coat of flat paint than it is to get great gloss AND opacity with a gloss paint. After all, a thick coat of paint hides detail.
The Future is self levelling and really lays down to a very thin coat - even several coats are thinner than most gloss paints I've used.
You can even apply the Future to just the areas to be decalled as well.
Good luck!