Not saying one is right and one is wrong, because there are few absolutes, but note that seminal vesicles are removed during surgery, so I do not follow the statement from the oncologist that radiation would treat that portion of the possible cancer better. As I assume you know, radiation as a follow-up to surgery is common, but surgery after radiation is problematic.
In addition, I believe that a lymph node dissection can be done before full-blown surgery or radiation (but not certain about that...) to determine node involvement. This is much more reliable than a MRI.
If I were in your shoes, I would get a third opinion, since it appears that you got two very different opinions from two very different types of physicians.
Good luck in your decision-making process.
Danman Bob, Born 1951
Nerve-sparing, open prostate surgery November 13, 2007
Gleason score 9, PSA 14; Biopsy result - 9 of 12 sticks showed cancer
Post-op pathology stated that cancer was confined to the prostate
Unrelated surgery January 2008 delayed incontinence recovery, which is now showing good signs of improvement (fraction of a pad a day as of late April 2008)
100 MG Viagra 3 times a week beginning December 2007
Osbon Erec-Aid Esteem manual pump for therapy beginning mid-February 2008
30 MG papaverine/1 MG phentolamine bimix injections beginning late April 2008
Five week post-op PSA 0.2, five month post-op PSA 0.1, next test August 2008