I have done both Judo and BJJ for years. I started with Judo before everyone on this forum was born LOL. I find that Judo guys are much stronger at the top game both on the ground and standing up. There is a base and strength guys develop practicing a sport where you throw each other's bodies as hard as you can while the other guy is trying to do the same to you. Judo guys are hard to move and physically the sport is faster and more violent than BJJ. There is no stalling in Judo. If you stall you get penalized. It might be fun if MMA had this rule.
BJJ guys are WAY better from their back. Judo guys always underestimate how good even blue belts are from the guard because Judo guys just do not work this position as much. So they get caught in triangles and armbars sometimes by making this mistake. (Fedor?) Also leglocks are not allowed in Judo and sometimes they get caught.(Pawel Nastula)
I love that in BJJ you fight until someone is submitted. In Judo I always complained that they do not let the fight go long enough on the ground. It drives me crazy that BJJ guys always pull guard and jump to their backs. I think BJJ schools should incorporate more stand up grappling. This is a huge weakness of BJJ. Plus stand up grappling is fun!
I will say that trying to get a BJJ guy to practice any type of stand up grappling is like trying to give a cat a bath. They just do not want to stand up. I can't get them to do it. Where a good judo black belt has probably blue to purple belt level jiu jitsu type skills because a lot of time is spent in practice on the ground in Judo clubs.
Bottom line. You really should know both. If you want a great example of Judo in MMA check out former Cuban Judo champ and BJJ blackblet Hector Lombard. He is incredible.