Polycotton is the one hybrid that I think does well. On any give trip, you'll enjoy up no matter the material. Night's are always cooler no matter the material and if the weather is hot during the day then you'd be outside no matter the material.
I have two polycottons, a way to big one that we use for family long stay camping and a smaller one (still big as we have a large family) that we use for shorter stays. I would only get polyester for backpack camping, as by design the main reason it exists is for its comparative lightness.
As for size, a good general rule of thumb I heard, is buy as big as you can pack, as it preferable to be able to have space and not use it than needing more than you have. However, as I alluded to above, it depends how long your stay is and what you are going to do when away etc.
Polycotton is my personal presence so I'd always go with that and I am happy to pay for the difference with an understanding that it will last longer (and is more readily repairable) and thus better on the long-term pocket but that is just me. My one suggestion though is to buy last seasons or the seasons before and the price will come tumbling down while the specifications aren't often much changed.
Good luck!
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