Honest intentions unless you have bad things to say about supply or the Navy. A good mixture of both is always a good bet. Never put any community down...just pump up the other if you can. Something along the lines of "While I feel I am an asset to the supply community, I see my talents and capabilities better serving the Navy in aviation." That's a nice, political way of saying "I can pick my nose for 23 hours and 59 minutes of every day and still be bored with the time I have left to do my job. I hate supply and everyone in it. I want to fly because those guys rule and I'm just not uptight enough to stay where I am and will end up in the future if I stay in supply."
p.s. Take everything I say with a grain of salt. I'm just Forrest Gumping my way through my tour on my ship until I finally get orders to flight school.