I have two 17's and two 67's. Your concern about weight is not an issue. I weight 235lbs. The main issue will be: Do your ischials and related soft tissue fit inside the steel frame that runs under the rear of the saddle?

I find the 17 to be excellent for all my riding, but the 67 is just a tad more comfortable. That's because at about 60-70 miles, I find that the 17 frame puts some pressure on the pads that surround my sitbones. The reason is that the 17 is slightly too narrow for me. However, this is only an issue on very long day rides.

The B67 is great for as long as I stay on the saddle. 100 miles is not even a test. Other things give out, but not my butt.

Springs:
They work on bad surfaces but you won't really notice them much in general because they are pretty stiff. The Flyers are nice. My next Brooks will be one.