Back in November of last year, I found and posted that Ubi had trademarked "Rainbow Six: Breakaway".
They have two trademarks for it.
Ubi has also registered the domain www.rainbowsixbreakaway.com
This leads me to believe that the PC version of Rainbow Six 5 will be titled Rainbow Six: Breakaway.
All trademarks for SC, GR, and R6 games by Ubi/RSE have been used, so I doubt Breakaway will be any different.