Something like this would be easy to make and easy to animate, also i would be able to change the facial expresions quite easily. If i was to use a design like this i would add more detail, maybe give it a uniform or props that it can carry. This will help to give each Alien a personality.
This Alien is a very simple shape and doesnt have lots of detail apart from its eyes and teeth, however i could easily creat different aliens in a similar style to this so that they would be easier to animate. They could also be in different colours and i could make them similar to this Alien with the front of the chest being a lighter shade of their body colour.