A few comments from my research and listening:
I think you'll find more than a few opinions on this - the purist will tell you the objective is to re-create the soundfield as it was mixed in the studio... ideally we'd have our rooms arranged with the same layout and speakers as the audio engineer who mixes it, that would be ideal.
Short of that, we try and set up our systems to approximate the mixing environment as closely as possible:
If you plan on doing any serious audio listening I'd go with direct radiating. DVD-As and SACDs are mixed with direct radiating all around, all at ear level. If you go with dipoles it will not create the exact stage as it was mixed (althought it will probably still sound just fine depenging on how much a purist you are)
Home theater is slightly different. The key with surrounds for home theater mixing is that they provide a nice ambience with occasional surround sweep without distracting you too much from the screen- directors, producers, and sound engineers mix to keep your attention on the screen. These are mixed to approximate a real theater setting. In most theaters the fronts and center are placed directly facing the audience while the surrounds are placed way over the audience's head - again to give that sense of direction but not to distract too much. In a home theater setup this usually means to place your fronts and centers at "ear level" while the surrounds are placed 1-2 ft above your head.
I've heard of bipoles in a 5.1 setup referred to in some high-end shops as the "poor mans" 7.1 - meaning the rear-firing section reflects off the back wall and provides a reflected sound from the rear, emulating the two rear surrounds.
I've listened to all setups and here's my tastes, yours may vary:
DVD-A, SACD: exact same speaker all the way around, all at ear level.
Home Theater 5.1: Fronts and center at ear level with dipole surrounds about 1 to 2 foot above ear level
Home Theater 7.1: Same as above, but with all surrounds direct radiating 1-2 foot above ear level.
I think the trick here is to listen to a representative sample of what you will be using the system for the most and make your decision on what your ear likes the most. Dipoles are usually 50% more $$ than their direct-radiating counterparts, so budget can be a factor too. But with 2K just for speakers you should have no problem picking up a great system any way you go.
~Josh
