Another factor is some vendors are now listing the raw input power, not the effective optical power. Diodes seem to be about 15% to 20% efficient so a 20W might be more honestly labeled a 3W.
I have 1.6W, 2.5W, 3.5W 1.5W & 15W modules. The 1.6W provides the sharpest focus, 2.5W visibly worse but used daily, 3.5w & 5.5w useless. The 15W was the cheapest, is close to 2W output power, and came with nice optics.