Hi SumZaf,
Quite often it is nice to know more about your diagnosis, such as T stage, N,M numbers, what percent of prostate was cancerous, but in your case as they have offered HIFU we can guess that T will be T1 or T2, HIFU would be unlikely to be offered if the cancer was starting to spread. Similarly your N and M will be zero, as if they weren't HIFU could not cure you. It is also reasonable to assume the percent of cancer is quite small as HIFU is a focal treatment usually aimed at a small area (though I thikn it can be aimed at the whole prostate).
Generally if you are offered a choice of treatments it is because in your case they are probably more or less equally effective, RT may have a slight edge on HIFU but slightly worse side effects so it's just a matter of balance.
RT may cause some damage to the bowels but it is unlikely to be very serious or long lasting. It may cause bowel cancer in years to come but that is actually very unlikely. ED may develop years later. I don't know much about the side effects of HIFU as so few on this forum have had it, but I think they are not too troublesome.
Of course you may be unlucky and get lots of side effects from either treatment, but I think you just have to go with the law of averages and assume that things will go OK.
I hope I am cured but I think it is wise to take a cautious approach and view treatment for prostate cancer as pushing the problem down the line which ever treatment you have.
I think if I were in your position I would take HIFU in the hope that it could be repeated if it failed first time and if that failed I could go on and have RT. But all of that really only applies to me with my attitude to risk and side effects, you may have a completely different perception to the problem than me, and there is no doubt that your perception is right for you and my perception is right for me.