Hi,
I looked at these two products two years ago, I lauding bringing in the Acumen team. At the time Acumen was clear that theirs was a companion product, not a replacement or competitor to Pimavera Risk Analysis (PRA). I believe the primary difference at that time was that PRA did Monte Carlo analysis and Acumen chose not to include that feature (as to not be in competiton with PRA). There was duplication of some features in both. What I found at that time was that Acumen allowed for much more granularity and fine tuning as well as having more diverse and robust analysis algorithms out of the box. Additionally, Acumen was very flexible with respect to bringing in WBS structures (activities and dates and durations, etc) from various sources (spreadsheet, P6, MS Project, eric) and tying these together of the analysis (which is fairly common on projects because vendors, partner firms, client, etc., use different tools for scheduling and planned progress). Lastly, I was impressed with the consulting capabilities of Acumen, as they were ready to sit down and work through the tool and scenarios to help you find the right analysis approach for your project / effort.
Cheers