Maybe this is a slightly naive suggestion...
...but would buying both not be the best option for the US Army? Common engines (I think?), potentially commonality in avionics, but different platforms for different missions. Not to mention, keeping the ability to manufacture military helicopters alive in two rather than one organisation (accepting that each involves multiple players). The SB1 looks like a much better replacement for the Apache than the V280, but on the other hand, the V280 looks very good as a Blackhawk replacement...