I think those three rings would probably "face up" really similarly, meaning that they would look similar on your hand looking down at it. The Harry Winston does have a slightly more squared-off halo than the other two, so it would look the most different at first glance.
The bigger differences are in the undergallery (the profile view).
The first is plain and a low set cathedral, the second is higher set with melee stones set in the strut, and the third is a medium set cathedral with a two-sided pave halo and surprise stones.
Another consideration is whether you want your wedding band to be able to sit flush against your ring with no gap. The first option has a matching band that sits flush. The second also has a matching band, but it is curved in order to sit more flush. The third's matching band does not sit flush.
It really just comes down to personal preference. I think that the first is probably the most delicate option and the third the "chunkiest". The second is probably going to look a bit bigger just because of the squared halo and non-cathedral style.
I think MoCo's version of the Legacy is not as chunky/big as the Tiffany version, but I agree with you about the halo looking too big for the stone. It's the main reason I decided against the Legacy. Have you seen MoCo's new version with milgrain? For some reason it seems to make the halo seem less big to me, but it still is a pretty chunky look. http://www.moissaniteco.com/cushion-legacy-halo-moissanite-wedding-set-w-milgrain-bezel-p-14924.html