Kate gave some great advice above. These systems can be very similar and customer service is key. Price does not always determine what value you will receive from a company. We have found that in this market, you can just pay for the name and not always get the best service available.
Since your father has limited eyesight, I would suggest that you be sure that the unit you buy has braille on the main console's buttons. Your dad may not know braille, but you can teach him simply how to feel the difference between them.
As far as the button style goes, you may want to consider the watch style over the pendant as it may be easier for him to find in an emergency. Just understand that if your father injures the opposite arm where he is wearing the watch, in an accidental fall or something, then he may find it difficult to press.