Quality of surround and ceiling speakers

"Can these be substantially less expensive than the front an center speakers?"

Yes! 

Matching center to fronts is very important. Try to keep the tonality, and the efficiency close to the mains, 

Surround: Directional cues, spatial cues, crowd noise, does not need much.

It looks like you’ve done your homework. Either of these look like extremely good matches. Because they are good total matches.

Now it’s a question of finances. I have found that the surrounds are less important then front and center. But it’s really a question of finances if it’s not much money to you then definitely get the better one. You won’t regret it or doubt the outcome. If you’re stretching to do it then you’re probably not gonna notice the difference. I find when I’m staring at a screen. I am diverted from the Audio enough that it is not as important as it is in a pure audio system.

We bought our surrounds nearly 30 years ago and we bought the highest level B&W 800 series. We have never regretted it and they sound fantastic... so it’s easy for surrounds to be a very long-term purchase.

You don’t want to go too cheap on the surrounds. You want something solid and durable because of how awkward it can be to reach them (you don’t want to have to replace nor troubleshoot them every six month), but as far as movies are concerned, there’s not much information coming out of there. You get your usual suspects: rain, helicopters, planes, dogs barking, birds etc. High fidelity is not really paramount and they don’t have to match the front necessarily.  Good luck. 

Spending an extra $6k for the 6 series for what is just ambient sound is fairly nuts to me and hard to imagine the 5 series won’t be perfectly fine.  With the money you save you could add two more subs that’d probably make a bigger difference anyway and take an African safari — that’s my perspective anyway.  Best of luck. 

It is very true that most of the time it is just ambient... rain, dogs barking... thunder. But once in a while it is much more. But good point / perspective.