I am going to buy a storage shed (10'x12' size) for my backyard. Saw there ar some plastic ones and metal (galvanized steel). So, I wonder which are more reliable/durable?
Thanks.

My steel one is 8 x 10 and has lasted through 10 years and a hurricane. I can't speak about other materials.

Edit: it is showing some rust and I'm looking to replace it with a stick built.

My steel one blew away, literally, during a wind storm. I replaced it with a plastic one and there is no upkeep at all. It still looks like brand new after 8 years.

The galvi rusts, no doubt about it.
If you don't mind how it looks, it is cheaper.

Plastic sounds longer lasting...it's only I am not sure how resistant it is on a snow? I do not want the roof to blow down when heavy snow comes. I believe steel ones are more resistant and durable.

Mine has a pitched roof. I bought it at Costco. It has been through quite a few snowstorms. The roof is attached to the base and it hasn't had a problem since we put it together.

I have a resin shed, that has held up well through more than 10 years.

Plastic will also age, and fade in sun,

Also, hard to repair, if there is damage.

I have seen them blow away in a storm also.

Metal, holds up a little better ( personal opinion)
But they rust and need repair, or painting at some point.

A wood siding with shingle roof, has a lot of benefits if looking long term.