121GW has by far the most functions. If the official firmware would have been open sourced I would have bought one. For example to put in some custom algorithms for sensors or even to fix bugs (It was quite new when I thought of buying it)
But a few years ago I thought a lot about what was the most important for me, and I decided it was accuracy, reliability and stability, and I bought the BM869s.
Data output was also a must for me, but the optical output of the Brymen is adequate. I would have liked a benchtop meter, but those are out of my price range, and they are also to big (Especially in depth, big empty boxes these days).
So as a result, I can build small and cheap specialized meters, calibrate them with my Brymen and then start measuring and logging.
But overall, the feature set of the 121GW puts it pretty much in a class of it's own, if it's feature set appeals to you, then buy it. If it does not, then get another meter.