Does an increase in cancer cell traction forces increase metastasis of the cancer cells?
You have pointed out in your question that two different research groups have differing conclusions about your question.  The answer is not known which is why they are studying it.  

Their differing results probably turn on how cell traction force is defined and the experimental system used to measure it.  It may not even be consistently ture - for example metastasis via the lymphatic system may be mediated by different properties than metastasis through the bloodstream or local tissue invasion.