Originally Posted by Danger4u2
Attaching at the axel provides unsprung weight to the bike. Any attachment to the bike subframe or frame will be sprung weight. I would think a low unsprung weight would be more stable off road, not changing the weight bias of the suspension fore and aft.
The trade off is that your rear suspension setup is reliant on the weight of whatever is hanging off the shock, which would now include the "tongue" weight of the trailer. In the world of high performance unsprung weight is far worse than sprung weight, as it compromises the ability of the rear wheel to track over bumps because of added inertia. However subframes are usually not designed for the kind of loads discussed and move the weight higher up on the bike.
If I had the skills I would find a way to hook the trailer to the footpeg mounts. It keeps the weight sprung, at a low, central point on the bike, better bias the front/rear loading etc.
The trade off is this method would be slightly heavier, more complicated, and less flexible in terms of moving the mount from one bike to another,