MIG will THEORETICALLY provide a stronger weld since you do not have stop/start locations or slag inclusions to worry about. However this is assuming the very best circumstances, including a welder who really knows what he is doing.
Under normal circumstances 90% of welders will make a stronger weld with stick, since stick doesn't require much skills or knowledge of the process to make a good weld.
There's no gas to worry about, and just one control knob. It's not hard for even an ignorant beginner to make a stick weld with passable weld fusion and penetration. ANYONE can figure out the correct settings for stick, through dumb-luck trial-and-error. In contrast you simply can't depend on trial and error with MIG.
The problem with MIG is most MIG machines out there are both seriously underpowered ( under 200 amps output.) Secondly, in my experience it's rare to find a welder that has the correct knowledge and experience to utilize the MIG process to anywhere near it's true potential.
The result of these two is it's common to find MIG welds with severe, unacceptable lack-of-penetration and lack-of-fusion defects.
Sadly 99 out of 100 welders never, or only rarely test th MIG welds they make for strength and weld fusion. They don't realize that the most important part of a weld is in the middle where it can't be seen.
To give you a specific example. A lot of welders think it's acceptable to use dirt-cheap 100% CO2 with the low-current short-circuit transfer mode, for welds on thicker steel plate over 1/8". A lot of welders also think it's OK to use one setting on their MIG machine for any and all welds. (they put a pencil mark there.) I've run across welders who have been in the business 30 years who do this.
In reality the performance of a MIG weld depends on: 1) Knowing the ideal voltage, WFS, and best shield gas for ANY given thickness of material. 2) having a machine that can back you up with enough current. It's a poorly known fact that MIG requires 50-100% more current than stick welding. Without 1 and 2, at best you'll be welding a lot slower than you need to, at worst you'll make a weld that looks pretty, but is completely defective.