Hi Cath - sorry to hear about your Dad. The radiation treatment of prostate cancer is my special area, especially the sort given from the inside, either as the seeds which are left there or the temporary high dose rate sort where nothing is left.
It is a really difficult choice to make as to what treatment to have, as all the options have really good disease control rates for men with early stage cancer.
Generally (and this does depend on the surgeon doing the operation, and the technique used) surgery has a higher risk of leaving men totally impotent, and of being incontient of urine. Recovery from surgery can take a while, and usually involves at least a week in hospital and a catheter for a while longer. On the upside, if they have a lot of problems with weeing already (like getting up in the night to wee, not having a strong stream) the surgery will get rid of these. Another downside of surgery is that if there was a recurrence of prostate cancer in the area, its really hard to treat.
Seed brachytherapy (internal radiotherapy) is done in usually 1 or 2 visits, only going in as a day patient, and leaves about a 100 tiny (about the size of a grain of basmati rice) seeds in the prostate. This then treats the cancer over the next 6 months. As the radiation is quite weak, theres no danger to other people, although in the first few weeks its best not to have children sitting on your lap for a long time (like watching TV together - a hug or sitting next to them is fine). Side effects are that it can make wee problems worse for a while, and some men will become impotent, but Viagra will work for them (with surgery it doesn't). If the cancer recurs in the prostate it can still be removed.
The Prostate Cancer Charity have really good information about all the treatments and their up and downsides