Choosing the right microcontroller
I think it depends on many things, like:

You mentioned already:


Capabilities/features
Familiarity (yourself)


Others can be:


Cost
Familiarity in your company
Expected support from the manufacturer
Swappability (how easy it is to later convert to different models when hardware requirements change)
Software/tools/IDE support (thanks to Peter Smith)


This list can probably be much longer.