I can only talk about concrete and hypertufa. How big are you making your pots? I have 3 foot arborvitae in pots where the bowl area is only about 1 1/2 foot square. These are outside year round. I've had no problem leaving hypertufa and concrete outside in my zone as long as it was planted with a tree or shrub. I also break contact with the ground during the Winter if the planter is sitting on soil. I shove a piece of wood shingle under one corner. Do people leave concrete planters outside in the Winter in your area/zone? That's a good sign you can leave yours outside.
There are lots of recipes on this site under the FAQ section of the hypertufa forum. You can use hypertufa for many projects. The peat moss recipe gives a browns/gray color and the peat moss on the surface of the planter will wash out over time giving you a pockmarked looked. Vermiculite will give you a sparkly look. Perlite gives you white flecks in gray background. Using white Portland cement gives you a more modern, white, white, white look. Before I forget, we are talking hypertufa and we use PORTLAND CEMENT not concrete mix.
Concrete mix is fine for some applications but if you want to carve or put imprints the gravel is a problem.
Hope that answers your question.

Thanks for the answer. I am mainly gardening so the intent is, at least with some pots to plant something which would not make it through the winter here we get -5Cunder freezing not often. I too find pots is the garden very decorative and you can plant something which for instance needs more drainage than you usually have.
I have a big plastic pot which would be a great mold. Some show that they mold inside, some outside that pot.

Concrete or hypertufa pots left out over Winter, that have soil in them, will freeze, the soil expands and can crack the pot. If you have the pots planted with trees or shrubs, the roots act like a cushion and help with the expansion of the soil.
If you want to reuse the plastic pot for a mold make sure the pot you are using doesn't have any undercuts. If anything is sticking out like a raised relief you won't be able to remove the mold unless the mold is very flexible.
Know what you mean about more drainage. Where I live it's mostly clay and a high water table. I had to put in raised bed vegetable gardens because of all the water.