Well, I also love satin. I find stretch satin flattering. Maybe I'm wrong but when I wear it I feel totally fab and also glamorous. I love the look with a denim jacket -- the opposites attract idea.
There's probably a reason you are posting pics of the dress on a mannequin and not your own self, and I'm just clueless because I have been MIA for too long. So I won't ask to see it on you instead...
I would say that the dress looks totally different in the two sizes. I have this same quandary myself when sizing up or down. I think size 6 may be what it's supposed to look like, as it's hitting the mannequin's real waist, while in size 4, it appears more empire because the natural waistline is obviously below the bodice. Question is, where is your real natural waistline? I don't know. Mine is up high so for me personally this would be a no-brainer and I'd choose size 4. I don't mind some va-voom; I think it is offset in this case by the bodice riding up farther for more coverage. I'd also personally like this look because I'd think that it would extend the length of my lower half.
But I'm not you so I can't say. Either look is great. They are just two entirely different looks.
Oh! Also I'd go with size 4, personally, because it drives me batty having the armholes too low. But I am prone to a bit of flab under the armpits, so I like higher armholes.
I hope that helps! I think the dress is quite lovely.