If you use prepareForSegue:sender:then you won't have as much to change if you later decide to trigger the segue from some control outside the table view.

The prepareForSegue:sender: message is sent to the current view controller, so I'd suggest something like this:

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
    // Assume self.view is the table view
    NSIndexPath *path = [self.tableView indexPathForSelectedRow];
    DetailObject *detail = [self detailForIndexPath:path];
    [segue.destinationViewController setDetail:detail];

In Swift:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    let path = self.tableView.indexPathForSelectedRow()!
    segue.destinationViewController.detail = self.detailForIndexPath(path)