Generally, yes, x370 boards are better for OC'ing, but that will vary by board. You need to review each board and user/tech reviews on the OC potential and stability. You can very well reach the same OC on a b350 as an x370, and save money with the b350.
Even within a manufacturer's board family, for example the ASRorck Taichi with 16 total power phases (12+4) and look at their b350 boards and find an 8+2. It's about budget and your desire for stability and reliability.
I went with the Taichi but I will never use SLi. I simply wanted the strongest board available that I felt could be a foundation for the next five years. I am very happy with my choice. You will find users who love their b350 boards as well and will never have...