I have back and knee problems and have 11 years on you. The big things to look at are the drivers and passengers seats. You need seats that you can move back all the way and lean back. Many rigs both A and C have some thing behind the seats and you end up leaning forward all the time. If you find the one you like spend time setting in the drivers and passenger seats to be sure you find comfort. Do not be in a big hurry.
You need room for your feet. I wear 14s and just need to be able to drive in comfort leaned back and feet able to move around. We also went with Chevy because at the time ours was built they had more foot room.
We went with a C because we liked the cab over because of the shade after spending 3 hours in an A setting behind the big glass windshield in the afternoon sun. Just to much for us. We use the overhead for storage. No way in heck we are getting up there..
Finding the one thats most comfortable for you is best. Bigger is not always better. In truth having spent time in both the one you find most comfort in for doing the things you do is the one you want.