A tougher blade that is suitable for mowing rougher grass, weeds, bracken and bramble and over rough ground where there may be stones etc that would damage a finer blade.
When peened it will also neatly cut finer grasses and soft weeds – a multi-purpose blade.
Choice of length depends on the size of area you want to mow and the number of obstacles to mow around. A longer blade gives you more “bite forward” and allows you to mow more with each stroke.The 55cm blade is better in more confined areas. It exerts less leverage on the snath, so causing less stress on the snath when mowing tougher vegetation.
All new blades should have the lacquer cleaned off the cutting edge to prevent it clogging the sharpening stone. See this post for further details.