I don't use heights and just briefly listened to a setup that had them, so I can't give a personal comparison. Opinions as to the benefits of the heights vary widely, as you can see from just the responses that were made here. I can say that if you have enough room behind the listening position(say, at least 4'), back surrounds provide a significant addition to the sound field with all music if a mode such as DPLIIx is used. Although there's relatively little discrete 7-channel material, that mode extracts useful content from both 2-channel and 5-channel material that is heard best with back surrounds and sends it to them. As Dean pointed out, ambience for height speakers isn't always present.
You should certainly get the extra QS8s and if possible try temporary placements as either back surrounds or heights.
