Hmmm ... good question. First, I guess you need to determine what you will be using the welder for the most.
If you are going to be doing a lot of auto sheet metal work, or just household repairs, then go with the MIG. MIG welding is much better for light-weight metal and is also very easy to learn. Steer away from flux-core and go with gas shielding.
However, if you are going to be doing a lot of heavier work, like maybe repairing farm equipment, then the SMAW (stick welding) is perhaps the more useful process. But stick welding takes a steady hand and is a little harder to learn, IMHO.
Not to say that you CAN'T weld light metal with stick, or weld heavy metal with MIG, but it typically takes more time and is more difficult. Skill then becomes part of the equation.
Either case, the small Miller AC/DC Thunderbolts (I assume it is about 230 amp AC and 160 amp DC?) are nice machines for light to moderate welding projects. Don't under-value the AC welding, I welded my old farm equipment together for YEARS with an AC-only machine.