Definitely spend for some higher-end shorts for those longer rides (45 miles and better). Also, thicker does not always equal more comfort: it depends where the padding is, what it's made of, etc. and the chamois has to match the shape of your "seat" where it meets the "seat".

I like the Castelli shorts ( of course, expensive) for the fabric, the non-silicone elastic, and supportive chamois. I also have a PI pair that are pretty good for long rides (3D chamois). If you are at the point of spending that much time in the saddle, shell out for some high dollar shorts. I have never put any goo on my butt, and have (albeit narrowly) avoided saddle sores.

It reminds me of my running shoes: I have 20 mile shorts, 30 mile shorts, 50 mile shorts, etc...