As a Miller guy-factory trained and very active in the aluminum welding industry...
Buy the Lincoln P-tig. If you can pony up the bucks, the 375 works even better.
All the good stuff needed for aluminum was changed, I think, with the Miller Syncro DX series.
They just do not run aluminum as well as the old Syncro 250 and 300. They run much like the old Syncro 350, a transitional machine that was not the best on aluminum. Tungsten spitting, arc rectification and erratic arc at times.
A loping, blubbering arc is also typical of the DX series-might have to do with the OCV-I'd have to look into this further.The Syncro 351 had OCV issues.
DC on steel, either machine will do well.Then, again, a battery charger with a tig torch would do well.
On the other hand-if all you want tig-either the Miller or Lincoln would be a good choice, unless very precise welding is required.
The Lincoln has a slight edge here., as far as transormer machines are concerned. The inverters ( Dynasty, Aerowave ) are clearly in Miller's corner for now.
To the rest of the group-I am definitely not trying to cause anothe Miller / Lincoln debate.
Most machines from either mfg are good enough to make a living with-that should be the biggest consideration.
Anyone who wants to talk tig, especially aluminum, please email direct- can't get out here as much as I would like!!