I am not sure whether you have vision problems or not. If you have perfect vision, you can just get fitover sunglasses to protect your eyes from harmful UV rays. However, make sure the fitover sunglasses can fit you properly and provide 100% UV protection. So you'd better try them on to see whether they are comfortable to wear. If you need prescription glasses to correct vision, you'd better go to see an eye doctor to get a pair of prescription sunglasses.
It is totally up to your favorite. Personally i prefer to buy prescription sunglasses rather than fitover sunglasses. That because I hate to wear two pair of glasses frames on my nose. And i afraid the outside sunglasses may slip off my nose when i take some sports activities. Nevertheless, if i stay in an environment that need to go outdoor and outside frequently, i prefer to fitover sunglasses for conveniences.