A (well) organized ride is usually easier than riding alone or with a couple of pals I find. ALong a metric century you will get to stop at regular intervals of say 12miles for food, drink, or just socially while alone, a rider tends to continue till tired which is usually not good. The rule is drink before you are thirsty, eat before you are hungry, rest before you feel tired,... alone, I never do what I preach !
Go for the metric century, you'll love it !