Xenos are controlled telepathically by a queen, which could be able to receive hallucinations. This may or may not turn towards Necros favor.
Necros are extremely durable, have impressive adaptive ability, and often work in packs, settling roles between themselves and using their abilities to strategize.
Xenos are also extremely durable, and stealthy. Similarly to the Necros, they work in groups, though more spread out. They strategize at a basic level- like a wolf or, as my friend put it, somewhat like wasps.
Necros can see the neurons in our brains, without ACTUALLY seeing. This means they can hunt any thinking creature, and understand the difference between non-necromorphs who are working against them, and non-necromorphs who are under the affect of the marker.
Xenos have an excellent sense of hearing, as well as being able to sense the heat of creatures, which gives them the advantage in fighting Necros, which produce a large amount of heat when created and retain this heat for a chunk of their natural lives. Xenos can also see eachother, meaning if a Xenos became a Necros, they would quickly come under fire from Xenos.
Physically, the two are the same- fast, strong, capable of withstanding environments that prove lethal to most other creatures.
The two are very similar, only differing in a few places. They can both differentiate from eachother. They have the same strengths. They use the same tactics. The few things the two don't have in common, is their way of reproduction and the Necros ability to turn dead tissue into a large bio-mass that would quickly cover their half of the world.