Aachen,
You came to the right place to get your answers. Either surgery or Brachy would work well; each has its advantages and disadvantages which you should become familiar with before making a choice. Get an opinion from an experienced surgeon and Brachytherapist.
This is a comparitive of the outcomes of different treatment options:
http://prostate-cancer.org/PDFs/Is14-2_p3-11.pdf
To answer your other questions:
There is a lot of data on secondary cancers: Brachytherapy has a zero rate of secondary cancers at 20 years, IMRT has a 0.5% rate and these are compared to surgical patients not to the population at large which is a valid comparison. You can search "secondary cancers" on the New Prostate Infolink for the various studies.
The question of salvage surgery after a failed radiation is a "red herring" arguement. Salvage surgery is not recommended, because of it's morbidity and is seldom used anymore. The most common treatments for a failed radiation is reseeding with a different isotope, cyberknife, cryosurgery and HDR brachy. All have the same effectiveness as salvage radiation after a failed surgery.
The advantages of surgery are for a young man the side effects have a much better chance of not being permanant. You also get a post op pathology and spotting a reoccurrance is easier.
The advantages of Brachy is that it is a 1 hour procedure and you can return to work in 2 days compared to a 2-4 day hospital stay and a couple of months recovery for surgery. Incontinance rates are near zero. Immediate complications from a Brachy procedure are zero with no record of anyone requiring a hospital stay. Complications from surgery are more common and include 1% death at 30 days from complications or infection.
The various qulaity of life studies conducted indicate that Brachy patients are much more satisfied with their ongoing QOL than radiation or surgical patients.
Review the recent posts on how surgery and radiation affected the members of this forum.
Regarding ED: if you now have some ED then both of the treatments will make it worse. You should discuss this at length with your doctors.
Many more will be chiming in with their take on your situation. Hope we can address all of your questions. As you noticed I'm biased towards Brachy, because I and many others on this board have had great results with few or any side effects from this treatment.
JohnT