I wouldn't advise taking either one unless you have clinical hypothyroidism, but what do I know?
If you are looking for a diet aid, then you want T3. I don't want to sound harsh, but if you don't know the difference between these two, then you should do a lot more research before you actually use it. T3 can be quite tricky and may not give you the results you are looking for. Then there is also the chance of doing some permenant damage to your Hypothalamic-Pituitary-Thyroid Axis or at the very least experiencing hypothyroid rebound and gain back all or most of the weight you lost.
I really think of T3 as something the contest guys would use. Us regular mortals should do fine with some nutritional discipline and cardio.