If you are new to Arduino the Uno is the best board to start with because most add-ons and most code works with it and it will be easisest to get advice here. Later, with more knowledge, you might decide to use a smaller board for your final project and keep the Uno for another project. That’s the simple part of the answer.
Checking if people are sitting on a bench may not be as simple as it sounds. What short of a bench are you talking about?
Is it effectively a flat timber plank with no soft covering and no backrest
Or ditto, but with soft covering?
Is it all timber with a backrest?
What style of backrest?
Is there softcovering on the seat? On the backrest?
You get the picture and I think a sketch of what you have in mind would help.
Are the people guaranteed to sit in predetermined positions or could a large person straddle 2 positions.
Is it full with 5 small people and also with 4 large people. (And by the way how many people can sit on it?)
I don’t think there is any obvious weight or pressure sensor that would be easy to fit - but I am certainly not an expert in that area.
Conceivably you could put load sensors under the legs of the bench and measure the total weight - but the weight of people is very variable so I can’t see that being a solution.
My inclination would be to use optical sensors even in spite of your concern about hats or coats triggering the mechanism. If you have a backrest the sensors could be high up so that small objects would be ignored. Light Dependent Resistors are very cheap, small and easy to use.