Answer:
On the surface, there is absolutely no difference between the two parties. The only difference between the two is their tactics.
The PSL focuses on shock politics, running candidates that have no chance of winning and marching in parades/protests that will not secure the change they seek.
The Communist Party USA focuses on coalition building, working with unions and Democrats to try to build a better society for all workers, rather than a shock and awe policy. However, with very little media attention or solid marketing strategy, the Communist Party USA is in a state of limbo: existing under remarkable obscurity.
If the two parties could work together and combine both approaches, they would become a juggernaut on the left. Furthermore, if the two could work in coalition with Greens, Unions, and small businesses (first, to rework the electoral process to make it easier for third parties to run; second, to promote a "working America" value-based, political campaign), they could remake the way our system works from a grassroots level.