Both of them are great dark types, but they have different battling styles.
- great defences
- less weaknesses
- good move pool/variety
- lower attack
- very low speed
- lower base stat total
- higher attack
- higher speed
- higher base stat total
- many weaknesses
For Scrafty, he is a very slow Pokemon, but can be helpful in so many ways. In single battles, scrafty has high defences which can help him resist other attacks, and finish them off after they attack. A good ability to pair this strategy up with is "Moxie". This battle style is good for singles, but can be used for doubles and triples (although it is quite easy to knock him out. For this, I would recommend EV training his Def. and Sp. Def - and you can also touch up on his Atk. if you wish. But, if you're thinking about making Scrafty a faster Pokemon, I definitely recommend using Intimidate, and it will lower the opponents attack - and help your defences. Your other two move slot are completely free of choice as he only has has two weaknesses. You can decide to put moves in that help defend it, or use it for stat-raising purposes.
On the other hand, Krookodile is a much more well-rounded Pokemon due to its higher base stat total. Any one of his abilities will be good for him because they all touch up on Atk. is some way or another. Krookodile has basically one attacking style which is attack your opponents to defeat them fast. He has great STAB moves such as Crunch/Foul Play for the dark-typing, and Dig or even Earthquake for the ground-typing (depending on your battle style). Even though Krookodile seems like a good Pokemon, you need to watch out for his weaknesses. He may have a fair amount of defenses, attack, and speed - but it is quite easy to knock him out. So far, his STAB moves deal with 0 out of the 5 weaknesses he has. In this case, I think fire is a good move type because it takes care of 3 (Grass, Ice, and Bug). The last slot can help with one of them, but not both. So, you can put one in to help deal with it, but it really doesn't matter.
Overall, I think you chose two great possible dark types. One one hand, Scrafty can help with your adventure with trainers, gyms, or even the elite four. Krookodile is quite similarly used for the same purpose, but he can be great for battles or PvP. I don't think you need an alternate dark type. If I had to choose, you should give Deino a try. He can evolve to Hydreigon which is a pseudo legendary. His stats are great in everything, and he has very good moves which can help you in battles a lot. You may not find him until later in the game, but you will before the Elite 4.
In the end it's basically up to your choice. I've used both an I think you got a 50/50 decision - because to me it doesn't matter. Good Luck! Hope I helped! :)
*Note: this is only for Generations up to 5. In 6+, there's the fairy-type which is a completely different story.