Yes, from a vibration isolation point of view the Studio USM is the only viable solution.
The key to effective shock-mounting is to have the most compliant direction of movement in the same plane as the diaphragm's movement. Anything else is a waste of money, but it's surprising how many commercial shock-mounts don't adhere to this simple but critical principle!
The diaphragm in a mic can be moved in two ways; either the mic body stays still and the varying air pressure moves the diaphragm -- which is what we want -- or the air stays still and the mic body moves taking the diaphragm with it -- which is what the shock-mount is supposed to prevent.
So thinking about a single horizontal pencil mic, it doesn't matter if the mic body goes up-down or side-side because neither of those movements cause the diaphragm to move in-out and thus don't generate an electrical output signal. However, fore-aft movements of the mic body will definitely cause diaphragm movement and an unwanted VLF output, so movement in that direction must be well-isolated.
If you look at the standard InVision 7 shockmount (below), the lyres are quite stiff with little deformation in the side-side and up-down planes, but are designed to deform very easily in the fore-aft direction -- and that's how they provide a very high level of effective vibration isolation in the plane to which the mic is most sensitive whilst maintaining good control of the mic so it doesn't become wobbly and suffer translational vibration modes.
The problem (as far as shock-mounting is concerned) with the S502 is that it isn't a single mic, it's two mics mounted at different angles, but its physical design means that it closely resembles the shock-mounting requirements of a side-address mic where up-down movements aren't a problem, but both fore-aft (and, to a lesser degree, side-side movements) definitely are a problem.
To provide reasonable isolation in those two planes simultaneously the shock-mount needs to have the lyres operating in two planes simultaneously, and the only Rycote design that offers that facility is the USM, because it has four lyres mounted perpendicular to each other.
Rycote's USM is a very clever design and is about as good as it gets for side-address mics, which would include the S502's ORTF array.
With a conventional ORTF array, using separate end-fire mics, the ideal solution would be along the lines of the double independent InVision set up like this which provides the greatest compliance on the same axis as the diaphragm:
But the S502 is a single-bodied mic and has to be treated as a single unit and, since the mounting is via the vertical stalk, it is essentially a side-address mic... hence requiring the USM mount.
I think, because the S502 weighs only a fraction over 400 grams, I'd go for the USM-L version with the most flexible red lyres.
However, my concern would be whether the 'stalk' of the S502 is long enough to reach the clamps before the cross-arm fouls the suspension, even if the mount is used inverted. I couldn't find any dimension measurements for the mic so I guess you'll just have to try it and see...
And purchasing a USM-L won't be a waste of money because you will always have side-address mics that need a decent shock-mount!
When you're using the thing, please do make sure you secure the output cable into the clip on the USM stand mount. That minimises the risk of any vibrations running along the cable from reaching the mic which is a surprisingly common source of VLF noise. Normally, I'd recommend using a very light-weight linking tail cable to help further, but that's not practical with the 5-pin XLR and stereo output cable for the S502.
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