I would say that if you're riding on rough terrain (e.g. rocky, boulder-strewn, heavily rutted, etc.), you would be shocked at what you can do on a full suspension bike. I know I spent the first couple of months with a F/S bike just riding over random obstacles out of sheer amazement. If you're riding mostly smooth dirt trails where your primary obstacle is occasional tree roots and ruts, then a hardtail is probably fine.