Unless your buoyancy, control and spacing are really good, which is unusual but not unheard of for a newly minted diver, it's hard to call any of them "easiest or safest".
Even for cavern you'll be in an overhead environment for much of the dive which can add a lot of stress to a newb diver, combine that with the damage that can be done to the cenotes running in to things and all the other things you'll be concentrating on (limited vis due to halocline, air consumption, equipment, etc.) this could turn ugly for you, the DM, a buddy or the cenote in a hurry.
This sounds like one of those situations where the opportunity presents itself at the wrong time and you'd be better off coming back when you've logged more dives. Better to err on the side of caution, enjoy the ocean dives and leave the beauty of the cenotes for another trip.
If you DO decide to give it a go I'd highly recommend doing some ocean dives with your chosen shop first and then asking them for an honest assessment of whether you've got the skills and comfort level for the cenotes.