Once upon a time... belt HP was the norm. Then PTO's took over as the main means of transfering power to impliments, so PTO became the norm. When tractors got larger and in some cases didn't have a PTO then drawbar power became a test standard for those models. In more modern times, since Kubota started advertizing their ponies by engine power the rest have more or less followed suit... although large FWD tractors have generally been rated by net flywheel power.
Most generally I refer to power in terms of PTO power for the models I deal with tho.