Man, what a topic! I love this.
I have always preferred the Telewave wattmeter. It doesn't need different elements. Not knocking the Bird, though. After all, it is the industry standard.
Back in the day, when I worked at a depot radio shop, we had, I think, 3 Bird 43 wattmeters, and a small box of elements. We used them almost every day. We didn't consider them to be precision instruments. Being the "Government" we had to send them once a year for "calibration" to our on depot cal lab. Once, we got one back with 3 of the elements in the same box with a note that said that this wattmeter was calibrated with these 3 elements, and not to use it with any other elements. We ignored that, of course. Like a lot of other commonly used instruments, the readings of a Bird are considered absolute. There will be no arguments. Having used network analyzers almost daily for quite some time, I have learned to be skeptical of instrument readings.