More and more Unity seems to be an interesting choice. Free, lots of assets, including those for filmmakers. I think they added a timeline feature recently so it's going to be more and more useful as an animation studio. It's much more difficult but at least you know it has future and it can look really good. Moviestorm is ok if you don't mind your movie will look like a game from 2004.
Moviestorm will have the future we users choose, The Movies, though not originally conceived nor designed as a animation tool, in spite of being older and inferior to Moviestorm from a point of view of their usability and flexibility, still enjoys a rather active and large user base.
Besides, using Moviestorm won't detract you from using other animation tools. Furthermore, I think most of the people interested in animation and machinima use more than one platform.
And no, I don't care about the retro look: I care about good stories. If the main reason to switch to another animation platform are the visuals, take into account that what is current state-of-the art graphics will look horribly outdated in, say, five years. On the other hand, a good story, especially if joined by a good production (direction, camera, lighting) work, will be fully appreciated not matter the time since it was created. Unless you refuse on principle to watch any B&W film because they look oldish.
Now if you're telling me Moviestorm suffers from a number of technical shortcomings, I'll agreed there's room for a good deal of improvements: maximum memory address space available, keyframing of animations, lighting system, ..., you name it.
But again, the decision is up to you.