Unless the client makes clear what they need, you risk getting it wrong.
Some Windows users might prefer DNxHD instead of ProRes. Some would prefer ProRes4444 rather than 422 if they’re going to have a heavy FX or Grading post workflow.
Personally if the client is professional they should be able to handle the camera masters and transcode as they see fit if their workflow requires it. If they aren’t professionals you certainly may chose the delivery codec for them but I think it should be clear, in writing, that sans their input, you decision is final.
At the point they sign the contract (or letter of agreement or email agreeing to the job) you can offer Native Camera Masters, DNxHD, ProRes 4444, ProRes HQ 422 and if one isn’t specific you reserve the right to chose delivery codec.
Keep in mind that uninformed clients or clients without the knowledge may respond “These Pro Res files are too big” whereas you could have given them 35mbit or 50mbit camera masters and get a response “We can’t play these” or “we can’t transcode these.”
BTW if they’re ducking because they don’t like spending the time transcoding than you charge them for your time to do that.
I should add that the best way to encode might depend on what your encoder can do depending on the flavor of XDCAM you shot with. Keep in mind that it might range from BPAV from XDCAM EX wrapped in .mov first or .mxf files and whether they are MPEG2 or XAVC.