If you know the exact dates on which you wish to travel and return, then a round-trip ticket would be the best option for you. Round-trip tickets often come with options to change the fare for free or a small fee depending on the ticket level so you can make adjustments. For complete convenience, if you don't know when you will return or just want to go one way, a one-way ticket is a better option and you can purchase a one-way ticket back when necessary.
Although round-trip tickets are generally more expensive, as you get two flights for the price, one-way tickets are often almost as expensive. This can depend on how many seats the airline has sold at the different pricing levels in the classes. One-way tickets give you more flexibility to search for cheaper fares on your dates -- for example using multiple airlines to secure the cheapest trip. Round-trips using the same airline do enable you to build up extra features such as frequent flyer miles.
On return tickets you can have the whole ticket cancelled if you do not fulfill one leg of the flight. This means that unless you fly the first leg then you cannot fly the second or return leg. One-way tickets give you more convenience about when and where you fly. It also means that the practice of buying two cheap return fares and using one leg on each cannot realistically be done.
Other options include the multi-city option. These tickets are useful for someone who will be flying to one city and returning home from another city. While you could buy two one-way fares for this this and cannot purchase a return, the multi-city option enables you to get what will most likely be the cheapest option.
