In the updated Mayfair edition of Agricola, I found two Major Improvement cards for the Well. One costs three stone and one wood (as it did in the original version of the game), the other costs three stone and one clay. In other respects the cards were identical. In a scan through the rules, I could only find mention of the Well costing stone and wood, and no indication of when or how the "stone and clay" card should be used - though I did spot a picture of the "stone and clay" card in one of the illustrations.
What is the purpose of the two alternative versions of the Well, and how do I decide which one to use?