Tai chi practiconers spend most of their time focusing on posture through the practice of forms, learning to walk and attention to breathing and relaxation. Aikido on the other hand uses attacks from a partner to learn calmness under motion and the intensity of physical attack. From this practice the aikido student learns the secret of internal power. These arts utilise the practice of weapons forms to further develop their understanding of movement and calmness. Aikido also offers the benefits of 'yuki' the use of ki to heal and I believe this is explored in some of the Chinese arts as well, in particular Chi Kung is practiced mostly for its health benefits
Aikido practices a number of blending exercises to learn to understand how to move and respond to applied energy, this is similar to exercises like 'push hands' that is often practiced in Tai Chi. Although many people associate Tai Chi with old people moving slowly in a park in the early morning, Tai Chi is also a serious martial art that takes many years to master. So to aikido is also a long road to mastery. Key differences in the practice if your thinking of doing aikido - we do a lot of tumbling. Tumbling or ukemi is the art of receiving a technique without injury, this enables us to practice aikido technique without injury to our partner because they take ukemi.