Sri Lanka vs West Indies - both teams won the first matches and today will be the second match. But, SL won match against Afghanistan and it is not so strong team. WI beat a strong team Australia. I hope WI got a good chance to win this match.
Another match is between Afghanistan vs South Africa - SA lost the first match even after scoring a huge runs so today they may go very strong against these Afghanistan bowlers. Lets see how Afghanistan will restrict these batsman. As of now, SA will be winner of this match.
Recently SL is not doing that great and there confidence level is very low. But there openers are looking good in this world cup matches and it is good sign for them so WI can't take them very lightly. SL main strength is spin bowling so if pitch supports spin bowling than mostly SL may give surprise to WI.
SA should be winning this match very easily but they may be targeting to win this match with good run rate to improve there net run rate. As you said, Afghanistan batting and fielding is good but they do not have a good bowlers to defend there runs. Hope they may do better in this match.