they're basically the same thing. I'm pretty good at decades, heres my advice
learn them out of a flat bank, then take them flat.
spin hard fs, kick the deck a little bit, so you'll meet it at 270
kick your feet up, as if your trying to touch your heels to your ass
keep your chest over your bars, and stay in line. like don't let your body go out to the side in an attempt to reach the deck. you'll only fall on your ass.
spot the deck and catch it when you're above it
once you get the rotation really well, then you can try decades flat. its the same thing just don't kick the deck and stomp it when you're over it. 360 downside whips are better for gaps or ramps, decades for flat. for me anyway