If you are asking the question of which micro is simplest I take it you have not messed with microprocessors before? If that is the case I would recommend buying an off the shelf solution like an arduino. That will allow you to focus on your application from the getgo, and less on the howtos of processors.
As for your project description, I see a red flag. Those membrane based flex sensors are not super repeatable, at least they weren't when I was working with them in the past. You more or less have to calibrate them often to get a consistent result. If I recall correctly this calibration involved depressing the sensors once or a couple of times to essentially warm them up. I could be crazy though, check the spec for your specific senor. My two cents, what you may actually want is a load cell, or a linear potentiometer rigged with a compression spring. In each case your sensor has a pretty consistent sensitivity to load.