I've tried, at various times,
voice only (cough, or "excuse me", or 'cycle behind you")
a bell
a dinky little battery driven box that makes a shrill whistle that is effing loud.
All have had mixed responses.
I've had horse riders thank me profously for calling out as I approach from behind. I've also had them look at me like I'm stupid for thinking that they couldn't cope with a pushbike. One actually said 'just pass will you'
.
Same kind of experiences with peds on shared paths.
Bells seem fairly neutral IME. Unlike the voice, most people (around here at least) instantly know it's a cycle coming, what they choose to do with that information seems to vary with their level of friendliness or chav rating. I've never had anyone criticise me directly for using one. When I've been out walking, I've found it the most easily recognized warning of a bike's approach. But then I am a cyclist.
The electric gizmo is far too loud to use on peds/other cycles/horses, but effective on close range cars. The drivers think they've run over a referee whose gone senile and is wondering around the High Street looking for a game to adjudicate over. At least they look for the source.
Conclusions? Well, much as I would like to think the bell is all I would need, it's not currently on my bike - the box of tricks is, for traffic use only. I'll continue with voice for peds and horses. It's a shame - bells are nostalgic reminders of more cycle friendly times, but they just don't carry the gentle message they once did.