I went from a 2.1 to 1.95 this season. The 2.1s are clearly better in the mud and wet conditions, or when the trail is loose. Overall, they are probably a better all-around size.
The 1.95s felt squirrely when I first put them on (I changed brands as well as size, so that must be considered). It felt like they were drifting all over, and downright scary in the mud. However, after a few rides I settled in with them. They are lighter, and roll slightly easier because you need a little more air in them.
Which is better? It depends on the trails you ride, the weather, if you race, your riding style and who knows what else. You'll probably see more difference changing tires types/brands than changing size.
My XC race tire is a Kenda Kharisma Lite, 495 g, 26x1.95. I ride Kharisma Krystals on my trail/training wheels. They wear longer, weigh 100g more, but have the same tread type.