Welcome to this forum but sorry for the reason that brings you here.
I was diagnosed when much older than you and had different treatment to that which you are considering. However, since joining this forum in 2008, I have read the stories of many men diagnosed and treated for PCa, some of them young like you. I hope my small contribution may therefore be of some interest.
Outcomes of men however treated for PCa can vary considerably as regards success in dealing with the cancer and side effects in the short and long term. So replies from men here may differ considerably from those you would experience for a form of treatment, although there are some generalizations as you have already observed. For example we have had a man who had a Prostatectomy by a leading surgeon but has been left permanently incontinent but this is rare. At the other extreme some men are dry very quickly. Also, in respect of surgery, some men go on achieve erections quite quickly with or without chemical or mechanical help whereas for others this is a slow process which in some cases may never be really satisfactory. Much depends here on the skill of the surgeon and whether one or both of the two main nerve bundles have to be removed as part of the op.
There are two forms of Brachytherapy, high dose and radioactive seeds either of which can if deemed advisable be supplemented with External Beam radiation. The risk of incontinence is lower with radiation as is ED for some time. However, radiation is usually preceded by Hormone Therapy which can continue for months or even up to 3 years after radiation as part of the radical treatment. This HT usually results in a loss of libido and erections tend to become less over time. When considering radiation one should ask also about the proposed length of HT because where this is introduced it will also have it's own varying side effects to add to those of the radiation, whatever form this takes.
It is more often the case that young men have surgery. This means men can sometimes escape some of the side effects of RT+HT or the small risk that in later years they may experience another cancer developing as a result of the RT. However, should it be found that not all the cancer could be removed by surgery, follow up RT can be given. (It is very difficult to remove the Prostate after RT and few surgeons will attempt it).
I would advise you to research the options you have been given in more depth and if you have not yet done so download or obtain a copy of the 'Toolkit' from the publications section of the main area of this charity.
There is no best treatment only what you feel is best for you. Whatever you decide, I hope it goes well.