I got my friend one of the Electra bells for her tandem... loud, sounds like a bike bell, and goes "ring" (actually "ding dong") rather than "clunk" (we're both musicians {she has perfect pitch}; it matters, lol. They had two in the store, I had to try both and pick the one that sounded best...). The Spurcycle bells sound nice, but they also sound much like one of the notification sounds on many phones, and I didn't want us getting into collisions while people nearby were all checking their phones after she rang.
The only problem is that we need better communication... every time she hears me say "on your left!" she rings the bell, even if it's not really needed (I pretty much got the bell for those clueless enough to not hear me say something). We've startled a few pedestrians that way... we might need to come up with a system where I let her know when she needs to ring (like when I see that me saying something doesn't work) but it doesn't have to be every time.