The eye level prism for the RB67 is really big, and fairly heavy. Whereas the waste level finder (WLF) is relatively small and light. I also find the view using it to be very satisfying
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The rotating back makes a WLF particularly good to use.
In addition to my WLF, I also have the PD version of the magnifying hood. It too is really good to use, especially for close-up work, because the TTL metering handles any issues arising from bellows extension. It is quite light, but it is bulkier than the WLF. If I'm using it with my favourite bag, its size means that I have to choose between it and a third lens. The PD version is the one to get, because it uses a modern silicon cell in the meter.
When considering this advice, it probably is useful to know that I've been using a WLF for decades, so the right-left reversal doesn't bother me particularly.