I think side surrounds depend on quite a few things. First, the size of the coverage area should influence what type you choose, budget will also influence things, placement compared to the seating area, and SPL levels should all be taken into account.
First, if you're trying to cover a wide area, then serious thought should be given to dipole/bipole alignments, with the expectation that if the design isn't well implemented it will more than likely end up sounding a little bit more smeared and less sharp than an accurate bookshelf. If the coverage area isn't that large a good bookshelf will probably fit the bill, with quality coming at a lower price as well.
If your side surrounds will have true side surround placement than any of the alignments would work, as long as quality and SPL standards are met to your liking. If placement isn't ideal, than it will depend largely on your ears to tell you what sounds best in your room.
Personally I prefer a higher quality surround speaker, but I also tend to listen close to reference level. When you listen at those levels you and you get the full effect of a movie soundtrack, a higher quality surround speaker makes a big difference IMHO. At lower levels I think it tends to matter less, but the louder you listen and the more you want to get out of a soundtrack, the higher quality you'll need, as with any speaker really.
I know for sure that when I watch the Abbado concert BD's I have (5.1 DTS HD Master), it just wouldn't be the same with lesser surrounds. Nor would many movies that really work the surrounds like Pacific Rim.