That's a good question. And good to hear what others suggest and are using.
There are a few flavors of OMT6. Did the arrangement you used add a near-spaced L/R pair flanking the X/Y center pair, or a rear-facing pair? Supercard directivity helps in those positions as well as in the center pair. It then gets down to where they are most useful.
One advantage of higher channel count OMT setups is that the mics help cover each other's deficiencies. A weak low frequency response in the supercards can be made up for by the presence of the omnis or other wide pattern mics. Off-axis pattern response deficiencies which sound not so great when used alone or in a stereo pair can get smoothed over by the mics, especially those facing other directions. In that way I was able to determine early on that I could get away with using good mics that were less than top-most quality and still achieve the results I wanted more consistently in taper situations, compared to what I was able to achieve using a single pair of top-quality mics on their own. Yes more mics in total, but less costly ones than I otherwise needed to achieve the same level of satisfaction in the end. Of course no problems using premium quality mics all the way around, use 'em if you got 'em. It's just that I'm unable to afford or justify doing it that way myself.
Based partly on that I ended up using DPA 4098H supercards in place top quality MG supercards. I think DPA 4098 run around $700 each new, ~500 used. I was fortunate to find a set of 5 secondhand for a very attractive price years ago. The are an odd format which I am happy with (I've posted plenty about them in these threads and elsewhere at TS) and use or have used them in all of the directional mic positions of my OMT6 and OMT8 setups, including the Mid channel of the center Mid/Side pair. Not saying they are the right choice for you and I will note that I've never actually run them in X/Y since I use a Mid/Side pair in the center. I swapped the M/S center that way using the 4098 out for an Audio Technica BP4098 stereo shotgun a few years back, partly to play around with an even more directional Mid, but mostly as an inexpensive way to upgrade the Side channel fig-8. I picked up the AT stereo mic here on TS secondhand. I think they run around $800 new, a good deal considering its really two mics in one.. which simplifies things in only needing a single mic-clip and cable. Its significantly longer though (see the "what a large Movo you have" comment above
).
^
For those reasons you might alternately consider a supercardoid X/Y stereo mic or a Mid/Side stereo mic with a sufficiently directional Mid for use in the center.
Of course a pair of super/hypers is nice to have for other reasons. They can be used to advantage in other positions in the array, used in other non OMT setups, etc.
