Both mics sit in a single Rycote suspension, using back-to-back clips. The wind screen has a larger diameter than the model used for the `416 alone, and the whole thing can be a bit heavy for handheld use. For that reason, I've added a Remote Audio hand grip to allow for two-handed use (which is handy for those extremely long shooting days). I've also rigged up a 2xXLR-3 to XLR-5 to further simplify setup.
I've used the `418s in the past. The `418s is a great choice if you're shooting production dialogue and the client is insisting it be recorded in stereo (for whatever reason), since its not that much heavier than a `416 and gives you comparable performance on the mid-channel. However, the MKH418s loses points for a rather noisey back-electret figure-8 side-channel capsule. This isn't that big a deal if you're recording hard effects, but I found that any atmospheres with a width greater than 45 degrees would really draw attention to the disparity in capsule quality.
That said, the `30 costs even more than the `416, while the `418s is around the same price as a `416. If you already own a `416, then the decision becomes a bit simpler. The `30 is quite a mic in its own right, and it will record deeper sounds than the `416 can (handling noise becomes an issue unless you engage the low-cut).
As for comparing various other stereo mics, I find that the M-S technique is quite well suited to hard effects recording, since a shotgun mic will give you such a strong center image. I've never found the adjustability of the stereo width to be that big an issue, though, since you can narrow any phase-coherent stereo pair by panning the left and right channels towards the centre. I've recorded some very interesting tracks using a pair of `416's in an 90-degree X-Y configuration, however the image had a bit of a hole in the middle. Some time ago I tried a Neumann RSM-191 M-S mic, but I was rather disappointed by the self-noise and susceptibility to handling noise.