Is flat land better than hilly land? Or is it better to have hills? This is a great question with a variety of answers that we will discuss.
Flat land
One thing that comes to mind about flat land is that all of it is usable for pretty much anything. If you want a larger tack room, or want to add a guest house, you have a flat area to use. Its also easy to see across your land. If a horse gets loose or you hear something at night, its much easier to locate your horse even if he's on the other side of the arena, or check on the horses from a distance, if you have a clear line of sight across the expanse.
Hilly land
While you can't always use every square foot of hilly land it does provide some variety in training atmosphere. You will want to give your horse some practice on going up and down hills before you take them on that fun trail ride, and having a few hills available where you work and ride can be a great help in getting your animal ready. Hills, and therefore valleys, also provide a chance for breezes to travel across the land following the elevation.