Jetphotos.net surpassed airliners.net in terms of photos on their database a month or two ago. Jetphotos.net has had more photos in their queue for a good while now.
Personally I find jetphotos.net easier to upload to as theres less categories to choose from to complicate things, and keeping track of your photos in the queue is easier on jetphotos.net as your queue photos are assigned queue numbers.
Also as mentioned before, airliners.net is a tad harder to get photos on, and they reject a good amount of cool shots due to small negligible reasons. Thus jetphotos.net offers a greater amount of shots for an airport such as Dublin and Belfast.
One last reason, Airliners.net tends to have more American photographers thus shots from the US. Jetphotos.net has More European photographers thus has more shots from European airports.
