What to look for in bike shorts

[KatieJean]

I've found that having my shorts snug--with good compression--really helps my leg muscles work for a longer time. My first shorts were just "yoga" clothes. Moved up to cheap-ish shorts from Nashbar, which were a world better. Then I got some Cannondales, and I will never go back to the ones that offer less compression. I don't do anywhere near a century (yet!), but even on my shorter rides, I can tell the difference. I'm guessing every little bit will help on your century, and snug shorts would probably be a plus (though of course everyone's different).
Good luck!

[HillSlugger]

The key for me for comfort on my metric century was ample use of chamois lube. I now use Buljum Budder. I rub it into the entire pad before I ride and add extra to my folds and rubbing points at every rest stop.

[Jiffer]

I read that too small is better than too big when it comes to cycling shorts. They should be relatively snug. When you stand upright, the padding should pooch out just a little, but when you bend over like you are in riding position, the padding should not stick out. Too big can cause more movement of the shorts, which could cause more chafing.

I agree you might want to consider spending a bit more for quality shorts to get you through a century, especially if you anticipate it taking a long time to finish. My two main pairs of shorts are the high end Specialized and Pearl Izumi. I make sure I wear those on my longer rides. Haven't come up with a preference for one over the other and the Pearl Izumi is a little less expensive, so you might look at those. Of all the things you buy to wear on a bike, the shorts are the most important.