There are several advantages to each and I feel they both can be equally effective to attendees, but there are many factors that will effect the use of them.
At HITEC, for example, they had a QR code on a large banner that provided information to the user about the app. The QR Code was large enough to get the attention of the attendees, as well as large enough to scan it with their mobile device from a distance. I also noticed that every time a user authenticated to the wi-fi, their browser was connected to the mobile site. This ensured that the information was always made available to attendees since they had to see it prior to using the wi-fi at the conference.
An app for Your Conference?
When using an app for your conference, you will have to ensure the attendees will download the app and refer to it when they are looking for the schedule and other important information regarding the event so make sure this is communicated well to the attendees. Question: How would you generate awareness about your app at your event?
I feel that either can be used just keep your audience and objectives in mind when choosing which one your organization is going to use. If cost is a factor, you may want to consider using QR Codes and a mobile site; they work very well together and the investment is very small when compared to creating a mobile app for multiple platforms.