First, if the senior has handicap priveleges then by all means use it for the special viewing that's available for this purpose for all these shows. There are many locations to pick from if you qualify for the special viewing areas.
If that's not the case then you need to decide what you are doing that day. As an example, are you just coming for the shows? If that's the plan then the closest place to the exit would be advisable because it's the least amount of walking.
If you are there to enjoy the park beforehand then your choice becomes from the point you are located at the time of the show to getting towards the exit for when the show ends.
Finally, I like certain viewpoints. For Wishes, I love being close to the castle or Main Street. For Illuminations, Italy is my first choice. Liberty Square is my favorite for the MK parade.
Also, please note you can do boat cruises for both Wishes and Illuminations but you are talking paying for that privelege. Also, you have to be lucky enough to get a reservation.
