Those are some nice mild temperatures.
Go with wood, the initial cost is cheaper. I also find them to be sturdier. Also if you are thinking the typical metal shed everybody's dad had in the backyard, they are pretty flimsy after a while and separate at the seams. Lots of insects that way.
Whatever you get paint it white; helps to keep things cooler in the summer sun.
Well, these temperatures are averages. It will go up to 27C today. But more often than not, it rains so I was wondering whether wood would be the right choice, don't want it to rot within a few months especially with expensive equipment in there. On a different note, how would you secure it? It's probably a lot easier to break into a shed than my house.