I think, in my opinion, it's more important how your equipment is set up. A poorly set up(voltage, feed rate) mig will not penetrate and get deep enough, but the same is true with stick and tig. I used a stick welder on mine with good results, but if I had the green for a mig at the time then that is what I'd have used. Sure a more powerful mig machine will give you greater capacity for what material thickness you can weld in a single pass, but they can also be set wrong and make just as big a mess as a smaller machine incorrectly set. To me, it's all about knowing your equipment and practicing. That's my 2 cents worth.
Everything I do is 100 percent tig welded. Its incredibly slow, and requires two hands, you cant get into some places that you can with mig, and the argon is expensive, but the welds are really strong, and the heat penetrates very well.
I built cookers using both Mig and Arc and have never had a problem. Like JayBird said it's all in the setup of your equipment.
When it comes to Mig, you need to make sure your machine is designed to weld the thickness of the metal you are welding. This is where I've seen people make mistakes. Their machine is too small to weld something like 1/4"
I use a small 110 mig welder on my cookers and have had no problems with them (YET!!!). I have welded up to 1/4 inch plate with mine.. I wouldnt want to try to weld anything thicker than that thou... On the 1/4 inch stuff I have to bevel the edges of the metal to insure I have good penatration but other than the fb and bp I dont go that thick.... And the 110 is good for when I do my automotive welding.
I'm the only one with a vizion... everyone else wears biphocals...
As the others have said it how your machine is set up, its capabilities, and the prep work you do on the metal. I have mig welded the parts to my backyard fire pit and it has seen some serious heat, no warping or cracking yet.
Is it normal for one testicle to be larger than the other two?
I have a Miller 200 mig at work and it works great if set right. At home I have a Miller 180 tig/arc welder and it works good for thinner stuff. For 1/4 or thicker I stick weld with 1/8 dia MG 500 arc welding rod and it is very easy to weld with and gives a strong good looking weld.
I also use a mig and I haven't had any problems. One advantage to using a mig is that you don't have to stop and change rods, you just pull the trigger and you can lay down a steady bead as long as you want as long as you don't burn through. It's like the other guys have said if you use proper techniques and a properly set machine you'll be fine with either stick or mig.