I prefer using TIG because it gives you much more control over factors such as: cleaning, penetration, speed and control of the bead being deposited.
MIG is in some ways simpler to do but you loose a lot of the control that TIG provides. If you only have a MIG machine (they're much cheaper and smaller) you can sometimes get better results (better penetration) on welding heavier aluminum materials by preheating with a torch.