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 issues. Unless you 'need' SLi and you 'need' to OC to the max, a b350 board will likely be more than adequate for you.