AdjeYo summed up very good the pros of Concentration (that are, the pros of Sentinel over Guardian) and I agree with him. However, I recently played my Focus Guardian abit, and I have to admit it feels much more comfortable to me than Concentration. It is less resource starved, and Combat Focus is much better than Zen, it just have a CD instead of building it with attacks, and it means you have a much easier time to catch up after a downtime (and I'd think you can also hit harder by basically reopening after a downtime versus the careful judgment of "how many Zen I have and which part of the rotation shall I begin" in the case of Conc.). Oh and it now also has better AoE due to Chilling Scream.