my Trigger was diagnosed this summer, and I immediately decided to go for the I131 treatment, as he is 11, and would face many years of medication, and possible side effects - your cat is very young to be diagnosed at only 6, so this applies even more to her
Trigger is currently on Felimazole, and on the advice of the mostly American Yahoo HyperT group I started him low, and increased after 3 weeks when he was still only just under the top of the range - he has just had another test and has dropped another 10 points, and more importantly no hidden kidney disease has been revealed
I thought I would have to take him to Langford in Bristol for the I131, which would have meant two separate trips for assessment and treatment, a three week stay, and a bill of £2000 or so - but have now found a newly opened (2011) facility in Wootton-Under- Edge, which is a two week stay, only one trip required, and a bill of £500 less - also being a dedicated animal treatment hospital, with no students and no research, I suspect the care will be all the better for it - and another big plus is that the vet in charge was happy to speak to me before hearing from my vet, something the bigger places will not do
I think 2 weeks out of a 6 year old's life is well worth it, as she should have 10 or more years ahead of her - a timid cat is not going to like the regular blood tests and daily dosing for all that time, even if you can keep her well all that time - my Trigger will not like being in isolation, nor will he like the 3 hour journey, but I am using an animal courier to give him stretch room, and I have no worries about it, as I would have had had I opted for an operation (not a good option for him anyway, as his thyroid gland is not very palpable, meaning there could well be thyroid tissue in his chest, which would not be touched by removing the gland)
do sign up to the Yahoo group - there are some people on there who know more about HyperT in cats than most vets in the UK, and collectively have a great deal of experience