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JimBI would agree with the chart that E94 is tougher than E106. Ebay has had the same one or two E94s for a couple of months now. In that list I think the E94s through E90-3 are all roughly equal in toughness. I think there is another big jump beginning with E99 and E100 to the end of that list. Those are all seriously difficult.
JimB
P.S. Ted makes a good point. In some sets, like E90.1, while generally it is not hard to get a type example, there are a number of very tough individual cards. The problem probably gets exacerbated by the number of set collectors of E90.1 who all need copies of those toughies.