I will not try to hide that I prefer Miller and I've used both but not the tig units. Both machines have ACand DC output. For welding Aluminum you'll need AC. By adjusting the current balance pos-to-neg you can adjust the cleaning action and penetration.
Both are capable of doing the job but the miller machine has been on the market awhile, the lincoln is new. Newer isn't always better and I prefer not to be a guinnea pig. Miller has a forum on their website www.millerwelds.com go to motorsports and click askandy, you'll have to register to ask any questions you also can read what others have had to say.
Don't go crazy on gas, a medium size cylinder is good. Straight Argon is what you'll want normally but by getting a medium size cylinder if you should need a helium mix for real thick material you can geta small one for that.
FYI: Tig is a difficult process to master and on the askandy forum is a man that is great with writen description of how to's, his name is HAWK. Aluminum is the hardest welding within the tig process and gives veterans some trouble now and then so patience is required and lots of practice. I would do all I could to do steel first in tee,butt,lap,corner, and edge welds then switch to Aluminum. Build on easier to harder rather than starting with the hardest first.
Email me if I can help more.
Good luck,
Sounds like you may not have a good gas coverage if porosity (holes) is the problem. Are you using solid wire or fluxcore? Some fluxcore wire is self-shielded and does not need an external shielding gas, however if you are using solid (mig) wire you definitely need the shielding gas to prevent the holes you are talking about. Technique will have an effect on gas coverage too, if you are leaning the gun too much you can lose gas coverage.
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