Working with a personal trainer, they'll be your accountability buddy and motivator, but sometimes can be difficult being that the motivation comes in a different form. On another note, it can catapult your performance just as much. Your trainer will start by evaluating your current fitness regimen, nutrition, lifestyle, and create a plan that is tailored to meet your life schedule, interests and goals. As you progress, you'll log your training in detail to allow your coach to analyze how you body is adapting to the workouts. It also allows your trainer to adjust the regimen according to your life; work, family, health etc. You learn how to train for you, and also gain the most from your efforts.
The list goes on and on, but if you're looking to improve, stay motivated, connect with others, or new to fitness, I'd start with group training. If your solely looking to improve your performance and learn how to adjust your personal fitness needs, go with a personal trainer. You'll obtain life changing results either way!!!