Metric fasteners can be a better option than standard hardware in many ways, but it can depend on the situation and if you have the option to use them. Metric fasteners are making their way into many industries because they are more universal and, in many cases, offer superior strength over standard fasteners.
One of the most beneficial factors that make metric fasteners appealing is that they are used worldwide, so no matter where you are, you can find the fasteners and tools you need to deal with them. Standard or SAE fasteners are primarily used in the United States and a few other places, but manufacturers are seeing the benefits of switching to metrics even in the US.
It was not too many years ago that every nut and bolt on a car made in the US was standard or SAE, but today nearly all of them are using metric fasteners. While your project may not be to that scale, using metric bolts can make it easier for someone else to work on later if required, and you can use fewer fastener sizes to accomplish the same strength while requiring fewer tools.
Metric bolts fall between coarse and fine thread SAE bolts, so the thread pitch and gauge can take advantage of both the strength found in fine threads and the vibration resistance of the coarse threads. The metric fasteners are also available in many different materials, so you can get bolts that are hardened or made from stainless steel, aluminum, and even brass if you need them.
Commercial coatings can also be applied with fewer interference issues on the threads, allowing the manufacturer to plate the metric fasteners with chrome, zink, or some other material to change their appearance while not affecting the fastener's performance. Since there are fewer size options with metric fasteners, the same fastener size is often used throughout an entire product. You may want to follow suit, making it possible to assemble your project with a single wrench or socket.
Metric fasteners are becoming much easier to find as they become more common. Most hardware stores, home centers, and even some big box stores have various metric fasteners to choose from. If you are looking for a fastener with a specific head design or a particular size, you may need to contact a fastener supplier to get them, but those are often very special cases.
Local vendors will often have the metric fasteners in different materials, so if you need the bolts to hold up outside or in a wet environment, you can choose a stainless steel fastener or even find metric nuts and bolts that are galvanized for corrosion resistance.