Here's the difference the way I see it:
MUST BE TRUEYou are going to be given a set of facts most of the time, but sometimes it is a clear argument. You must give an inference based on the information given.
Example: Tonight the Lakers are playing the Celtics for an NBA basketball game. All Lakers and Celtics games are on ESPN and no other network. All NBA basketball games on ESPN have high ratings.
There are infinite inferences you can make. But two easy ones here are that tonight's Lakers and Celtics game is on ESPN, and that CNN will not be hosting the Lakers and Celtics game. Both statements I listed must be true.
STRENGTHENYou are getting an argument and you are asked to pick a statement, if true, which will make it stronger. In the example above, it's pretty flimsy. One way you can strengthen this argument is that all Lakers and Celtics games have high ratings. You pick the statement that strengthens the argument the most without having to stretch beyond the scope of the argument.