The best type of posture brace for you depends on your unique needs.
If you are currently having sore back and neck issues, or have developed acute postural issues, like rolled shoulders, then a standard posture brace might be your best choice. You will find that these braces are more firm, and give you a higher quality support to guide your back into a proper sitting posture.
A posture shirt will work if you are just trying to stop beginning posture related issues. Posture shirts have the added benefit of being discreet, resembling normal clothing. If you feel like you are starting bad posture habits, the posture shirt will provide you with the moderate support you need.
Posture bras are designed specifically for women, and are an especially good choice if you are suffering from bust related neck and back problems.