Wire mesh used in construction helps strengthen the concrete forms around it, but this does not mean you want to throw any old mesh into that pile of cement that you're spreading around. You need something that will add the appropriate amount of strength for the specific project while retaining other qualities appropriate to the project type. For example, a layer of wire mesh in a driveway may not need to be very flexible, but wire mesh in a steel-construction building (which may still have concrete in it) needs to be more flexible for seismic survival. When faced with the options, the results you want are what will lead you to the best options.
Gaps Don't Necessarily Decrease Strength, but They Can
Wire mesh for construction comes with varying sizes of gaps in the mesh grid. Some of these are square and some rectangular. At first, you'd think the larger the gap, the weaker the mesh, but that's not the case. You can have small gaps with very thin metal (almost like a screen door, where the mesh is very fragile) and large gaps with thick, strong metal. If strength is the key quality you need, look at the size and type of metal involved, rather than the size of the gaps.
Material and Corrosion Risk
The metal used in the mesh affects the corrosion risk, too. Stainless steel is resistant to corrosion and will be "stronger" in that respect than plain iron, which might end up rusting if the concrete around it cracks and lets water and air in. You can get wire mesh using galvanized steel and other types of metals, too. Keep in mind that wire mesh used in a concrete driveway will almost certainly be exposed to moisture and air at some point, either through soaking rains or cracks as the ground settles and heavy vehicles continue to park on the surface. Stainless steel will be a good choice if you're constructing a concrete driveway.
Remember Flexibility for Seismic Strength
More areas are starting to experience quakes due to wastewater disposal techniques and some fracking. Areas that previously had little to no seismic activity are now seeing shaking that wouldn't do much in a state like California but that can cause minor damage in places like Oklahoma. New construction needs to be stronger and better able to handle the shaking, even if it seems like the quakes are mostly minor. That means wire mesh used to reinforce walls needs to be more flexible to bend with the swaying of the wall during the shaking. A stiff wall is actually less strong during a quake.
Talk to a supplier to find the right kind of concrete wire mesh for your project.