They both have certain advantages, however I would suggest Judo.
I cannot explain in words how frustrated I get watching fighters in the cage who are grappling on their feet, but never go for body throws, hip tosses, or leg sweeps. Things I ddo and think about doing on instinct. IMO Judo includes a great many techniques that are the perfect counter to not only most grappling clinches, but also the Thai clinch(not to say other forms of grappling dont include these, I'm just not educated in those). There are also many ground position techniques which are just as useful as BJJ, however I have found there are significantly more, at least from my training, movements designed to finish off one's back, not just sweep, which is advantageous against wrestlers.
There are many aspects of wrestling that are also very important. The single and double, top control, guard passes. These are all major groups of techniques that are essential to MMA groundfighting.
As said above there are possitives to both and you should train all the major aspects in both if possible. Starting with only one, go with Judo IMO. It's groundwork focuses on fighting off the back as much as on top, and it adds a lot of versatility to your takedown ability.
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