I think it ultimately just comes down to playstyle. Maybe certain matchups, too. But I think all versions of his arts are good.
His default Arts are versatile, you can usually go with any stance so long as you can avoid the downsides. Not much to be said.
Hyper Arts sort of requires Shulk to be more on the offensive, and limits his options within those 6 seconds. Very powerful, but so are the downsides, so enemies can just as well seek opportunities since Shulk only has 6 seconds to make a move. Or they can just avoid you for 6 seconds and that's that. IMO it's the most predictable and avoidable of his choices, but packs a massive punch. Also, Shield basically loses its entire purpose.
Decisive Arts on the other hand, aren't meant to be used for opportunities. It assumes a more calculative, perhaps strategic playstyle. The Arts are stronger with the same downsides, that's a pretty big plus for me. Of course the main issue is you can't switch, but that's why you put some thought into each choice, rather than just the buff you want. Buster is stronger and you have time to calmly make the best of it while playing safe, isn't that awesome? Shulk's recovery is good enough to always make it back on stage unless you're hit off-stage again, but if you're in vanilla, Decisive Jump makes up for the activation time anyway. Getting stuck in Decisive Smash after a KO isn't that bad when you can KO even earlier with it, and if anything, you can take advantage of your opponents trying to finish you off before it runs out.
Getting stuck off-stage with Shield is the main issue with the Decisive, and I don't have much advice for this one. But it will protect you a bit more from getting in that situation, and from even more damage. IMO, use Shield at mid-percents to take longer to reach your KO percent (rather than stretching your KO percent), and use Buster later instead, to try and rack up damage with zoning as they try to finish you off.