Should I do color grading in FCPX or round trip do DaVinci Resolve?
Use Resolve, it is far superior to FCPX for grading.

This is my workflow, since I always shoot in RAW with the Blackmagic Cinema Camera.  I know there are ways to do this using XML, but I've run into issues with that when I segment a larger clip in FCPX and then bring it back to resolve it will then have each of those clips as individuals, instead of the one larger clip.


Import the RAW DNG files to Resolve
Add all clips to my timeline (their order doesn't matter)
Apply a Blackmagic film to rec709 LUT and perform some basic grades
Export as individual clips. The format and codec you use are up to you, but for editing I use 422 proxy


Ensure that you export your clips using the source filename and save to a folder titled something like "Edit Media" or whatever you'd like


Import those clips from the "Edit Media" folder into FCPX and perform your edits.  Once you're satisfied with your edits,  go back into Resolve and perform the final color grades of the clips you wound up actually using in your video.  Render them at a higher quality (how high is up to you) and overwrite the clips in "Edit Media"
Open the project again in FCPX and render your video.  The clips should automatically be overwritten with the new ones you just exported from Resolve, as long as you keep using Source Filenames