Ditch the Tri shorts, and get a pair of cycling-only shorts or bibs. Tri shorts are great if you're planning on swimming and running on the same day, but are a little comprimised for just cycling events.

My experience is that the pad in any decent mid-range short doesn't make as much of a difference as the saddle. Shape, width and profile can vary widely and one may work for you better than another. I can put up with just about anything for 30 miles, but more than 50, and i can feel it. I rode the same century back to back years, with the same kit, and similar amounts of training. The first year, on my 'old' bike with a classic (too-narrow) 'Turbo' saddle, i was constantly in and out of the saddle on the last half to get relief; the next year on the 'new' bike, I had a Specialized Romin and it was never an issue. Best thing i can say about that saddle, is that I didn't really feel it.

Different saddles work for different people. Some guys swear by Brooks, I happen to like the Specialized Body Geometry line with the 'split-tail'
It does take some trial-and-error to find out what works best though, and the only way you know if a saddle will make a century, is to sit on it for 100 miles.