5.1 speakers, what to add to upgrade to 7.1

So I posted this earlier, but haven't gotten much of a response, so I decided I would make it a little more general.

Currently I have bookshelf speakers as my rears in a 5.1 setup. I am working on setting up a dedicated theater room, and am thinking about adding 2 more.

If you had your choice, would you:
1 - put bookshelfs for the rears and dipoles for the sides
2 - Bookshelf speakers for the sides and dipoles for the rears
3 - matching bookshelf speakers on both sides and rears

Thanks guys

the only thing i would put back there is speakers from the same line, Infinity Beta.

The room is 20ft long, 12ft wide for half and 9 ft wide on the other half. couch will be right in the middle, with the screen on the 12ft wall, and the rears would be on the 9.

I can make a diagram if you would like.

the bi/dipoles would be Inifinity Beta es250, wich can go either bipole, dipole, or 2 seperate channels depending on how you set it up.

I already have the bookshelfs that would go in the rear.