I just switched from monopoles to dipoles and to be honest, I'm not that happy with it. I think monopoles are by far your best bet if you listen to mostly music. All surround music is mixed in a set-up that uses monopoles for the surrounds. I'm thinking of thowing my old bookshelves back into my system to be able to listen to my multi-channel SACDs happily again.
The only factors that might change that opinion could be room size and placement. I have a small room where my surrounds end up equi-distant from the mains and at the recommended location. If your surrounds aren't going to end up like this, you probably could experiment with dipoles.
One other thing to throw into the mix. M&K makes a type of speaker called a "tripole." It combines the attributes of a direct radiatior with a dipole configuration. Supposedly, it provides the best of both worlds. If I had the money, this is what would be in my system. You can get a pair of their smallest tripoles, the K-4s, for around $500. (That should tell you about my current audio budget.
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I wish I could be more conclusive but it really comes down to speaker placement and what works in your room. If you're sticking mainly with music, however, monopoles are the definitive recommendation. I ended up trying out dipoles because I got a pair pretty cheaply. Good luck and have fun!