In a nutshell, O1 steel is easy to hone and a very fine edge achieved relatively quickly. A2 steel is a little more difficult to sharpen but retains its edge for longer. The downside is that because of the different grain structure the A2 won't be quite as sharp as the O1. What does this mean in practice? If you’re roughing something to size or planing abrasive exotics, then A2 is probably the better choice. However, for fine finishing or the pure pleasure of working with the very sharpest of tools then go for the O1 option.