If you go to the player instructions screen, the game will highlight what player attributes are "needed" for that particular role. So, for example if you set your player to "poacher" the game will highlight his finishing, among other attributes.
As far as which striker role will score the most goals, that is a question that has no definite answer because it all depends on the tactics you use, the players you use, and how you instruct them. No role is really "better" than any other.
Sweeper keeper is a GK who tends to come off his line and sweep up balls that are past his defenders. This is useful if you have a really high defensive line, thus you would need support if the ball gets over your defenders heads, behind them, etc.