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Yep, I would go hardwood or composite if you are planning on keeping it any length of time.
In terms of the frame, are you concreting the posts into the ground? If so I would think about what wood you are using there, as standard 4x4 posts will rot out faster than anything else, you can slow that down a fair amount by using post savers or slow it even further by using a harder wood for the posts. I used 8x4 oak sleepers ripped in half, and then put post savers on as well for double protection.
So in terms of decking, smooth wood, composite, hardwood, all good.
What I'm planning is a fairly substantial timber base, as if I were putting a garden office on it, but putting decking on it for the moment.
If we extend the house, we'll probably stick to decking, if we don't extend the house we'll build the garden office.
It it were up to me, I'd put fairly cheap decking in case we only keep it a year, but my wife is rather worried about safely.