I agree with what Darth Simon said about... well, pretty much everything. He just got to it before I did. Yoda apparently knows that he has two options, but Luke apparently has more potential as a Jedi than Leia, so he'd prefer to train him. Why would you go with your "backup" plan when your number one is right in front of you? So if Luke stayed to complete his training, Yoda would be able to train him to be powerful enough to defeat the Empire and wouldn't need Leia. He tells Ben there is another, because if Luke gets killed while trying to rescue Leia, then he can train Leia. How he knows that Leia will be survive if Luke doesn't, well... okay, you have a point there. I always have been disappointed with the way the "another" was touted as this big thing yet it never really comes to anything. With the way it was talked about, you would expect this "other" to come to Luke's aid at the big showdown in ROTJ and save the day.
But I don't see the point in saying that Lando wouldn't stand up to Vader if Luke didn't come. Luke had nothing to do with Lando's decision to step in. It was when things were going completely different than Vader had promised. Han was as good as dead, and Leia and Chewie were as well if Lando didn't step in. If anything, Luke's arrival hindered their escape because they had to go back for him.
And something that I know for sure no one else has addressed in this thread... Luke was talking about the Imperial Academy when speaking of Biggs and Tank. They joined the Imperial Academy, and that's where Luke was aiming to go as well. But in that Biggs on Tatooine scene that was cut out of the movie, Biggs informs Luke that, the next chance he gets, he's going to defect to the Rebel Alliance. Who's to say that Tank does the same thing, though? As for Tanks's whereabouts when Luke is reunited with Biggs on Yavin? He could be an Imperial officer for all we know!