Neither of these matches should have gotten this far, if you ask me. So I find it crazy that one of them is getting even further.
Both of them are on the list probably more for the spectacle surrounding the matches than the matches themselves.
Rock/Austin is remembered fondly because it was the main event of maybe the best (and certainly the most beloved) Wrestlemania of all time. It was also the two hottest stars of the moment facing each other right at the peak of the hottest period in wrestling history. It was a huge match. And on the night it delivered: it was pretty damn enjoyable. But it's far from the greatest match ever. It was probably only the third best match on that particular show.
The storytelling wasn't all that great, the finish was famously a mistake, and the bell didn't ring signalling the end of the match until at least five minutes after the climax. All of the kickouts by the Rock at the end of the match dragged things on unnecessarily and the match finished on a whimper as a result. I'm not saying the match was bad by any means, I just don't think it can be held up to the standard of most of the others in this tournament - including those that it's beaten in previous rounds.
Punk/Cena is remembered fondly because of the awesome Punk promos leading up to the match and the intriguing stipulation that WWE went on to completely ignore after the match. The storyline here centered around cementing Punk as a massive main event star, and creating change in the WWE. Ultimately neither happened.
The match itself was good. Not flawless though. Both guys can be a bit sloppy in executing their moves, and that was on display here. The commentary was also terrible, as the commentators refused to put over many of the aspects of the story that Punk had been addressing in his promos. But the story told in the ring was very good. The crowd reaction was awesome, and there was just a real buzz about the match. I wasn't a huge fan of the finish, but it was serviceable.
I'd definitely go for Punk/Cena as the better of the two matches. It's probably better by some distance. But I don't think it should have gotten this far either. Lesnar/Guerrero definitely should have beaten it in the last round, in my opinion.