For details on the differences between the two types of queue, see Lion’s answer.
Both approaches will work. However, NSOperation is mostly needed when more advanced scheduling (including dependencies, canceling, etc.) is required. So in this case, a simple
DispatchQueue.main.async { /* do work */ }
will be just fine. That would be equivalent to
dispatch_async(dispatch_get_main_queue(), ^{ /* do work */ });
in Objective-C, which is also how I would do it in that language.