Sambo is a mixture of Judo, wrestling, boxing, kickboxing, and, they say, native Russian martial arts. There are various kinds of Sambo. The two most common are sport sambo and combat Sambo. Sport Sambo does not allow chokes, but it does allow leg locks. Combat Sambo involves chokes, and also includes techniques that would be banned in MMA, such as eye gouges, fish hooks, and skin tearing (tearing apart the nose or hands are some that I've been shown). Usually a place will teach both Combat and sport Sambo.
Systema is pretty diffirent from sambo. There are also diffirent styles of Systema. Some have techiques and are harder, while others are very soft with few, but are rather based on movement, breathing and relaxation. The style you're most likely to find in North America is the Ryabko style, which is the later of the two. It includes grappling, striking, and Jujutsu/Aikido style joint looking. It has a emphasis on weapons (disarming and use of). It covers all these topics in a fairly unique way. In the end I think Sambo and Systema training together would be excellent.
"Seek not to follow in the footsteps of the men of old; seek what they sought."