The factors to take into consideration when deciding where to ride your horse are: where you keep your horse and your level of experience with horses. Other aspects such as whether you want to have a pleasure ride or to train your horse will also factor into the location of where to ride your horse.
Growing up in the country I had access to a vast area of dirt roads that lead to open areas for hacking and trail riding. Both the roads and the country posed their challenges, but I was never short of a place to ride. I did however have to face my fears and leave the safety of our property if I wanted to ride.
If you are learning to ride or if you have a new horse, the best place to ride your horse is in an arena. An arena offers a place of less distraction than a road or the trails; your horse is less likely to get a fright and panic. If your horse does panic there is usually help close at hand.
Riding in an arena should not be considered boring; it is a place to learn trust and confidence for both the horse and rider. Any horse no matter the breed or intended discipline to which he is being trained can benefit from being ridden in an arena.
When both horse and rider have a level of communication where the rider feels confident and in control then they can venture to ride outside of the arena. With the use of the word control I mean that in the event of "fight or flight" you will be able to react immediately with good riding skills and to regain your horse's composition before he panicked.
The advantage of a big stable yard is being able to ask other riders where they ride. Not only will you learn specific trails, but also the hazards along those paths, such as barking dog that never fails to respond to a horse passing its property.
A horse box is a wonderful means of transportation and certainly opens up the world for places to ride your horse. If you do not own a horsebox and can not afford to purchase one, you can always ask the help of other riders. Some riders will share their horse box just to have a companion on the trail; others will lend or rent you a horse box.
With a horse box you can travel to parks where there are specific trails or bridle paths for horses. You can travel to competitions where, if you entered the competition, you can ride your horse in the competition arenas. You could also take your horse to the competition and choose to ride him around there, in order to expose him to the environment.
When deciding to own a horse for riding, always take the time to explore and investigate places to ride before you find a place to stable your horse. That way you when you have to choose between the lunge ring or the arena, you are not bored and upset because it is the setting you chose. If it is however the only option you had at the time, start saving for that horse box.