Linktropy 8510 can simulate one link through each of four port-pairs. Netropy N61 can simulate up to 15 links through a single port-pair. The price is the same. But there are some differences:
Netropy N61 can emulate 15 separate links compared to 4 for the Linktropy 8510. If you need more than four links, Netropy is the obvious choice.
With only one set of GigE ports, the Netropy N61 is limited to 1 Gbps in each direction aggregated over all the WAN links. The Linktropy 8510 can simulate 1 Gbps on each link. If you need to simulate multiple 1 Gbps links, Linktropy is the better choice.
If neither of the above criteria determines the choice, then it becomes a matter of feature set and preference. Linktropy is designed for absolute ease-of-use while Netropy is more feature-rich to meet the needs of advanced users.