I do have one of the Mercury boxes as well. Over the years, I've developed impressions of the RCA sound vs. the Mercury sound.
I think Mercury does a better job of picking up the ambient sound of the hall. RCA can be more dynamic and precisely imaged. I've described it before as the difference between seats in the 10th row compared to seats in the 25th row. Some like Mercury, some like RCA. But I don't think there is any comparison in the actual vinyl pressings. RCA's are cleaner, IMO.