Seems to me this issue is very much āhorses for coursesā, coming down to what you feel comfortable with.
I havenāt tried EndNote: out of my price range, I felt, though I can no longer be sure of that; and at the time it seemed to be very much tied to word and be Windows-like from all the comments I read, and I have been a Microsoft-free zone (apart from the necessary evil of an MSN account, though I use Adium) since OS-X came out.
I have never got on with Firefox: purely a matter of personal preference; I have been an OmniWeb user since OS-X came out, and though I have always had Safari to hand, I prefer Opera as my second string browser; so Zotero was not an issue.
So it came down to Sente vs Bookends for me; and it came down once again to simple personal preference. I found on trial, brief I admit, that I didnāt get it with Sente but I did with Bookends. So I invested in Bookends and have never regretted it.
I have also to admit that Iām anything but a heavyweight user, but as time has passed and all of them have continued to develop, from what I read, I canāt detect that much differences in ability between them. I think itās only if you used all four āfor realā on an extended basis that any differences would really show up, and whoās going to do that. Iām not in a position here in China to test out things like searching distant libraries, and am not doing anything that requires that, so I canāt comment on such matters.
And along with Druid, if I had been in an environment where I could have got EndNote for $13/year and had been able to use it, I would have taught myself to be happy with it, Iām sure.
So I stick to one, Bookends. It suits me; the library file is in a folder which is synchronised between my two computers through SpiderOak (originally the now inaccessible DropBox), and it can do far, far more than I need it to do.
Mark