I am not an expert at visual effects, so apologies for inaccurate information. 
Visual effects is the process by which imagery is created or manipulated outside the context of a live action shot in filmmaking and video production. To sum this up, you could say that it’s used to make the story look sort of like a realistic environment?
To be honest, both are just as good! Whichever one you are more comfortable working with, you can do. Tappable is more advanced than the manual menu, so if you’re used to working with advanced coding; tappable would work nicely.
There’s no required length for story entries, but I think that 10-15 min is a good average of most entries I have read in past contests.
Hope this helps you! :]]