I think the way this question is phrased, because of the use of the word ROOKIE, the bigger bust would most DEFINITELY be a linebacker, not a quarterback. Nobody, and I mean, NOBODY, expects a rookie QB to do well and lead his team to a championship. Even Peyton, who threw for more TDs I think than any rookie QB ever, he also threw a ton of interceptions and his team went 3-13 his rookie year. Ben Roethlisberger clearly fell off in his production toward the end of the season, and in the end, they won the Jets game despite him rather than because of him, and they lost the Pats game because they depended on him too much.
On the other hand, if you get a linebacker with the #2 pick in the draft and he doesn't even crack the starting lineup as a rookie? That may be considered the biggest bust in NFL history, at least for one full season it would be. We're talking Tony Mandarich bust.
I mean, do you consider Phil Rivers to be a bust? He hasn't played a down of meaningful football yet, but I would by no means consider him a bust. But, if you got a rookie linebacker at the #2 spot and he didn't even start, that would be a bust, most definitely.
Now, on the other hand, which position has potential over their career to be considered more of a bust, I would even argue that they are about the same. Linebacker isn't normally the kind of position worth investing a #2 pick in, there is rarely a player THAT special that you think he's the next Lawrence Taylor. On the other hand, QBs can go #2 in the draft just by default (see this year) because of how important the position is. If you set them both to zero production, as in the linebacker either never successfully cracks the starting lineup, or sucks when he does, and the quarterback never successfully wins a starting job and/or sucks when he does...well, the fact that there's a whole lot of instances of the latter happening, and not too many instances of the former happening, I might even say that the linebacker could be considered a bigger bust because you not only reached for a position that isn't generally considered important enough for the #2 pick, but you failed when you did do it. Using the #2 pick on a QB, even if he does end up failing, is widely considered a more worth it gamble if there's any indication that your #2 pick linebacker may end up not worth it.
At best, IMO, this question is flawed because what you're really getting at is which position drafted could negatively impact a team more, and the answer is easy a QB. But the reason is not necessarily related to the investment of a #2 pick. If a linebacker turns out to suck you know pretty fast and you can discard him and find someone that can play pretty decently pretty easy. If a QB sucks you're never quite sure whether the light just hasn't clicked on yet, whether he does actually suck, or whether you need to be doing something differently on offense to take better advantage of his areas of strength.
But as rookies, a linebacker who doesn't start but got selected at #2 would be considered BY FAR to be a bigger surprise bust.
