Traded up this past spring to a Trek 7.3 FX from my old Multitrac 700. Love the extra gears for the hills around here. Do a lot of 25+ mile trips (the 2 close towns, Ashland and Bayfield, WI) and was finding my saddle to be downright painful. After a few trial runs I drove 60 miles and found a bike shop that had a "sit bones" measuring device. Quite simple when one thinks about it. Anyway, it worked. Went for my first ride (with a sore butt) and was amazed at the difference. Not only that but the saddle I needed and bought fits me to a tee AND it was on sale. My recommendation is to skip the trial and error method and find a shop that has one of these or simply sit on a piece of memory foam and measure the distance between your sit bones. What a difference.
As an aside: for my 7.3 FX some nice 700 x 28 road tires will make all the difference in the world for road riding. I went from 38s to the 28s. It's like a whole different bike.
~Gary (Washburn, WI)