Do you need to match back to already shot film? Then 24p is your answer.
If you're shooting this and expect to lay back to film then 24p is also problably your answer.
If you're shooting and expect to make digital copies then I'd stay as far away from 24p as possible. 23.98 is what you need.
From Canon's best practices PDF, page 7 ( http://www.learn.usa.canon.com/app/pdfs/white_papers/EOS_C300_Best_Practice... ):
EOS C300 cameras allow users to choose two frame rates that at first glance appear to be “24P”.
For conventional digital video recording at “24P”, users normally want to actually be shooting
at 23.976 fps. However, for video recording which must be precisely tied to existing 24.00 fps film footage, there’s a completely separate fps setting. It cannot be over-emphasized — the 24.00P setting is NOT for general-purpose video
recording. DO NOT set 24.00P unless you deliberately intend to match video files with existing film footage which was shot at 24 fps in a film camera, or unless postproduction staff have specifically instructed camera operators to use this setting instead of 23.976P.
So unless you need true 24 for a specific reason I'd steer towards 23.98 as that is the standard.
You may want to adjust your 24p footage then by running it through Compressor and making it true 23.976 footage. FCP can do mixed frame rate timelines, but it sometimes hiccups and causes headaches. Or if you plan on going out to other apps (Color, After Effects, Resolve, Smoke, etc.) they may not all be equally capable of handling a mixed frame rate timeline.
I'd take the hit now and adjust your 24p footage. Otherwise, it might come back to bite you in the end.