There are a few posts on the board about the backs, and relative difficulties. Recruits are the most common by far. Napoleon-backed cards consist of the same 150 players as the Recruits, and are much less common. I believe there are several of us (both on and off the board) actively collecting them.
Broadleaf backs are in the tougher class of cards, and have no overlap with the the players found with Recruit/Napoleon backs. In that case, the relative value is based on who is on the front.
Here's one reference about the set, including info on the backs.
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Mike