Depends on what your goals are. If you want to do a D.O. Fellowship, be a D.O. administrator (higher up politics) in the AOA, then you will need to take the AOBFP boards. You also get the board certified in "Family Practice and Osteopathic Manipulative Treatment" designation if you want to market yourself who does OMT.
As far as relative difficulty, in my experience taking both boards, the AOBFP exam is far easier than the ABFM. The OMT component is not difficult as well. One case, one problem, give a few items on the differential, and then perform OMT.
As far as CME, there is an easy solution. Take an osteopathic medical student, and all of a suddent it's 40 hours of AOA CME Category 1B credit per week for a total of 160 in a month rotation. Pain management CME is also easy; just use the DO-online website, do them online and you'll get your 12 credits no problem (if your state requires it).
For my case, I did an ACGME residency, wanted to do a DO sports medicine fellowship, so I took both. I use OMT, so it's nice to be able to refer to my board certification.