I would go w/direct unless there would be a chance that you'd use these speakers for a 7.1 application in the future (having 2 sets of dipoles, one as side surrounds and one as back surrounds).
Cost would also be a factor for me, aren't usually directs less expensive than a comparable dipole?
Dipoles/bipoles have speaker drivers opposed, i.e. on opposite sides of the cabinet. Dipoles, the drivers operate out of phase. You'd mount them towards the side of the listening position (for sorround use), with the drivers pointing towards the front and rear, with the null (where the out of phase drivers are canceling each other out) pointed at the listening position. This creates a VERY diffuse surround, as almost all the sound is being bounced off objects in the room, and makes for a very diffuse, hard-to localize ambiance surround.
Bipoles have the drivers operating in phase.
There are also some others that, for instance paradigms ADPs, are partly bipolar, partly dipolar. They are dipolar except for low bass frequencies, in which case they are bipolar, so that all the bass doesn't get canceled out.