Forgive my ignorance, but I have absolutely no idea what WMC is. If the W stands for windows, then I'll have to go with Apple. It's one of the few brands these days I can say I have complete faith in.
Few microsoft apps and pieces of hardware I've used have worked properly. Even my Xbox, which I love, has the annoying habit of ALWAYS resetting its internal clock if it's disconnected from a power point for more than 24 hours.
I don't know much about Apple TV right now, but I'd be inclined to side with Apple on this, for the reason that I can rest assured it'll work, and work **** well. Converting movies wouldn't be a problem, because there are way too many movie formats these days anyway. Converting to one format can only simplify things.
Also, since we only use mac in this household, it'd just make compatibility a whole lot better.