I did the I131 with my Gizmo, she was 14 when it was done and it worked. Only problem is you have to keep in mind this process does compromise their immune system, just like it does in a human. So if your cat has any other medical issues, other than diabeties then you need to be sure and discuss this with your specialist. I lost Gizmo about 6 months after she had the I131 to cancer. What the vet thought was a degenerative problem with her bones was actually a form of cancer and the I131 weakened her system enough for it to take over. With that said, I am not trying to scare you, just want you to be aware it does compromise the immune system as I did not know this until after she had her treatement and I had lost her. That was 9 years ago.
I now have a cat, Beemer, that has been taking the oral meds for hpyerT for the past 3 years. The only issue we have encountered was while trying to regulate her dose she became anemic. With a little tweaking that resolved and she has been doing great. She has a prescription card through Walgreens and we get 90 days of meds for $12. Beemer is a very timid cat and does not come out for strangers, so leaving her in a facility for 4 days was not an option. One day at the vet to have bladder stones removed really wore her out, she sat on her bed in the kennel and would not move not even to potty. Plus after losing my Gizmo, I was to afraid to risk it again. And I have to add after Frodo was diagnosed with diabeties and we changed his diet to low carb, it made a world of difference in her. For a 14 year old cat she has lots of energy and tends to remind me of a kitten the way she rolls around and plays. I really wish I knew way back then that the dry food I was feeding was kitty crack as the low carb food has been the best thing for all my fur babies.
Also you are right the meds to do not cure the hyperT they regulate it just like it does in humans. I know people who have had the I131 done with their cats and just like others have stated here everything was fine. You have to go with what you feel is the best treatment for your Spot, just be sure and have all the information when making the decision. And part of that is how/if the diabetic condition is an issue.