Magnetrons are used to generate long microwaves. Low frequency magnetrons are more efficient than high frequency magnetrons. Microwave ovens have magnetrons which operate at a frequency of 2.45 GHz and have efficiency of about 70%. Magnetrons operating at 915 MHz frequency have efficiency of about 85%. Magnetrons cost about $0.1/W.
Gyrotrons can produce short microwaves (< 3 mm) which are easy to focus into a narrow beam, but their efficiency is low (15%-60%). The maximum frequency is about 170 GHz. Gyrotrons cost about $1/W. The best gyrotrons are made by a Russian company named Gycom.