On Bi-Di being the best for surrounds,...??? well I think you'll get a lot of recommendations,... you will also find many people prefer direct/monopole.
Personally I like my Bi-Di side surrounds but if I moved on to 7.1 I would go with direct radiators in the back and keep the Bi-Di on the sides. Some like all four surround speakers to be bi-di, and some like all four to be direct.
There are also various versions of these speakers, ie; Bi-Polar speakers, Di-Polar speakers, switchable Bi-Di speakers, switchable Bi-Di and Mono, and like I have, a sort of hybrid Bi-Polar Woofers and Di-Polar Tweeters. I'm sure there are other variations as well.
So, there really is no "BEST" for everyone, but their is a "what you like best". Your room and your ears will likely dictate what sounds best as far as Bi, Di, Mono surrounds. In short,...go with what sounds best to you. If possible bring the bi, di, direct speakers home, try 'em out, keep what sounds best and return the rest.