Whichever you choose, take your time. Stop at every rest stop, eat a snack, get off the bike and stretch or put your feet up.
When you overhear the folks talking about blowing past the first rest stop because it's "only at mile 12" or whatever, ignore them. You'll last a lot longer, and (particularly if anything on your fit is off at all) your back will thank you.
I did my first century back when I was 46. At that time, my longest ride had been 40 or 45 miles on super-flat terrain, and 30 miles of hilly terrain (my one organized ride at that point). I just wanted to say I'd done a century, so I signed up and pushed through. Elevation was pretty middle-of-the-road for WA (4100 feet), but compared to SoCal, where I could ride 40 miles and come back with 300 feet of elevation, it seemed endless.
I felt great for 50 miles, a little creaky for the first 5 minutes back on the bike at every stop after that, and around 65 or 70 miles, my back started to kill me. Cleared up around 90--but that was a long 25 miles.
A couple centuries in, the pain started hitting later and later, and now I'm fine all the way through, even in years where I start with a century. I probably should have started slower and done the 50-mile route first, but getting one under my belt made me want to do more, and now I'm aiming at a Super Randonneur series next year, and hopefully a 1200k at PBP the following. Will I make it? No idea, but it's fun, and it's another stretch goal like my first century. There's no wrong answer for you. Just take it slow and trust your body at that rest stop where you have to make the call.
And have fun!