I think the structure of reddit is probably better suited to internet communication nowadays (read: better memes) than SDN, which is based on the traditional forum model from the early 2000s. For example, there's a pretty standard way to make a wiki, a much better thread management system, and an upvote/like system which is less cliquey than SDN often is. But SDN's age gives it a lot of things that reddit can't really compete with (yet), like the presence of many med school faculty/adcoms. The community on SDN may be better right now, but Premmedit has gotten exponentially better in just a couple of years. Aside from adcoms on SDN and the school specific threads, I think reddit is probably going to become a more and more popular place for premeds to get information.