In one situation, you are starting a new (one way) conversation. In the other, you are responding to one. If we respond to someone, it's pretty normal that it is placed on 'the end' (it follows). I believe that in our mental model, things end on the bottom, never on top.
I would recommend that you always put new items on top, except if the reason for the new item's existence is based on a previous item. In that case, it has to follow, not lead.
Benny Skogberg also has a valid point in letting your users decide how they want to access information, but I wouldn't place that much effort in such 'luxuries'. Focus on getting the initial concept (combining the news feed and instant messaging together) as good and as early as possible to your users and get feedback.
Good luck with the application!