So the consensus is that in modern English at least, it's /weiß wörße/ in both BE and AE.
German speakers should note that both /s/ sounds are unvoiced, so it's not an exact rhyme with Börse (except in southern regions where /z/ does not exist).
BE speakers should be aware that, as AndreasS explained, it does not rhyme with 'cursor' in any variety of English in which final R is pronounced (rhotic R). That includes not only the vast majority of AE but also many BE regions outside southeast England (Scotland and Ireland, for example). So it would be better not to say flatly that it rhymes.
I don't recall ever having heard anyone actually say /weißi/, and I would find it bizarre, though I have heard /weiße/ and it doesn't bother me, possibly because at normal speed the difference is not that perceptible. I believe the two-syllable version is still listed in some dictionaries because it used to be BE RP, back in the days of old-fashioned ultra-anglicized public-school classical Latin, with long Latin vowels pronounced as if they were long English vowels, e.g., Lat. 'pater' and 'mater' to rhyme with Eng. 'later' and 'waiter.' Remnants of this pronunciation survive in both BE and AE, especially in legal terms, such as 'habeas corpus' (long A as in Eng. 'hay') or 'prima facie' (long I and A as in Eng. 'prime' and 'face').
AFAIK, though, it's modern classical Latin that has C as /k/ and V as /u/: /uike uerßa/. Nowadays among English speakers, that pronunciation, the one Mary knew as Protestant, is used in most Latin classes, in contrast to Italianate aka church Latin, which is used in singing by both Protestants and Catholics: /wietscheh werrßa/. But no one would ever use either of those for a Latin phrase adopted into English -- so, yes, Kimi, you would be a total geek if you did. (-;
A couple of links for anyone interested in Latin as pronounced in English:
http://d.a.nicolas.free.fr/ucla/ling10/page31.htmlhttp://p211.ezboard.com/fwordoriginsorgfrm20....(plus other links in that thread)
See also this parallel thread on Latin phrases in English:
related discussion:Latin phrases in EnglishAnd finally, a note for German Girl: it's proNUNciation. Everyone who missed it, your homework is to write that and say it aloud five times. (-;