Bi-pole sit behind the listener and shoot left right and for forward....di-poles sit like regular rears but fire sound from left and right and not forward. With both you have the sound that bounces off the walls to give you a better wrap around.
Department of Heuristics and Research on Material Applications
Personally, I've always prefered dipolar speakers over bipolar. To me they sound less obvious, and more enveloping.
But I prefer basic direct radiating speakers in a 7.1 or 5.1 setup over either dipolar or bipolar surrounds. Especially for multi-channel music. Just personal preference, but many share my sentiments. Make darn sure you test all three types to see what you like best, preferably in your home if possible.
Which one would you use for a side (6.1, 7.1) surround - Bipole, Dipole or regular type of speaker?
Note that bi/dipolar both pertain to home theatre applications: Dolby Digital, DTS, THX, not to multichannel hifi, SACD, DVD-A where surround speakers are supposed to be the conventional type, and "equal" to the front speakers.
If you have a wall in the back and have room for it, a bipolar tower will give you the benefits of a bipolar surround (diffuse sound) while also providing the benefits of a direct radiator (localization) because one of the driver sets is pointing forward. If you listen to a lot of music this might be the best option, otherwise you might want to consider direct radiators.