I know most of you people will instantly say Vector, but really, that might not be the only answer.

Vectors:
Layers - you can add things behind others and still keep what's behind the other shapes.
Easier selecting and grouping - You can just select something just by clicking on it, also you can select a certain amount of shapes and group them by selecting the area they are in.
Not pixelated - This is one of the main reasons vectors are considered better.

Bitmaps:
Easier shapes - Not only can you use shape buttons, but you can use lines to enclose a shape and it can still be filled with colour!
Separating - You can separate shapes by selecting different areas of the shape.
Images - Bitmaps can be images from you're files and can be recoloured if they are not turned into vectors.

say [So...] for (2) secs
ask [Which do you think is better?] and wait
say (join [Are you sure it is ] (answer)) for (2) secs

I don't know! tell me when you find out!

This forum is for asking questions about Scratch, not hosting a survey.
Also, please don't blockspam.

1. Please don't blockspam.
2. My vote is that it depends on your art style. Some people (Like me!) find Vector a lot easier to draw with. Others find Bitmap easier to draw with. Some people like the look of pixels, others (Like Me!) prefer the look of smooth shapes.