The high res one is more uncommon, but don't think it's hard to find, either. Just in comparison to the regular video card. I think in terms of being the most common, it's Mac II (Very common) > High Res (Somewhat common) > Display 4*8 (Harder to find common) > Display 8*24 (Somewhat uncommon) > Display 8*24GC (Uncommon) > Display 24AC (Rare)
That's just from me browsing eBay listings for the past few months. If you had to sell one, sell the regular one, but neither are anything special as far as I know.