Ditto to what Jason and Dan said. I know that with League Park and Cleveland Stadium postcards, you can find the same basic picture on what are technically four, five or more different postcards.
Here are examples of a League Park PC that's essentially the same shot:
If you look through some of these, you'll see more examples (check out the second and third rows on Page 2):
http://www.freewebs.com/indianfever/...pcsgallery.htm
As for one variation having more value, I suppose some are tougher than others, but my guess is you'd have to be pretty hard core to know which ones.