Nancy -
I have been on both and am doing another Silversea in November as you saw in the Silversea section. Although the Silversea pricing indicates it is good through June 30, 2007 - I believe it is capacity controlled and that itin you selected could be sold out.
I was on a Caribbean itin in October 2006 on Silversea and did not find it stuffy - I am in my early 40's and the trip I was on was a bit more scaled to the younger than I expected.
If you can get the Silversea discount - I would grab it. Take the money and run. I am sure you will enjoy the Silversea experience.