Full suspension mountain bikes are much more comfortable, enjoyable then hardtail mountain bikes! The only down side is that there is a little extra weight and slightly less efficiency, but its is all worth it in the end! The difference in a hardtail and a full suspension is that the full suspension has a back shock. The way the back shock works is that is has three pivot points, when you go off a jump the pivot points move and the shock compresses and it is a easier landing.
Hardtail mountain bikes pedal better than full suspension, especially on smooth terrain. Hardtail mountain bikes are a little bit lighter than full suspension and require less care! Hardtail mountain bikes are much easier to get up hills like I said earlier, full suspension bikes are heavier and harder to get up hill.