Here's my experience with two of the brands mentioned: I had a Craftsman MIG which was made by Century. It had a decent Tweco torch, but was not a very good machine. I sold it and bought a Miller 135. That's when I realized that half the skills I had developed were actually needed to overcome the crappy performance of the Century/Craftsman. With the Miller, I [i]rarely[/i] screw up a weld.
Also had a Craftsman stick welder. Not sure, but may have been built by Century also -- definitely not Lincoln like some of the older machines. It was crap so I sold it.
Now I'm sold on Miller. Have no personal experience with Lincoln, but my friend, who is the service tech for a local dealer, says they're good too. Lots of units out there in commercial service.
Hobart & Miller are the same company. Hobart is to Miller as Black & Decker is to DeWalt (home shop grade vs. professional grade).
Anyway, you can't weld aluminum directly to steel that I know of and it's kind of impractical to use a stick welder on aluminum. Most any MIG can handle aluminum wire to some degree, but a spool gun eliminates a lot of jams (which can be VERY annoying). Aluminum and steel can also be welded with an oxy/acetylene torch, although not to each other.
I'm sure you know that although initial cost is higher, a MIG is [u]much[/u] easier to use than stick and it's generally more versatile. If you know you're going to do a substantial amount of aluminum work, you may want to get a machine with a spool gun connector built in so you don't have to buy an interface.
Now, where to get your welder? I'm totally sold on [url=www.welders-direct.com/merchant2/merchant.mv]Central Welding Supply[/url]. Dale is terrific to deal with. Their prices are VERY LOW and shipping is FREE. I ordered a new Miller Trailblazer engine-driven welder from them and paid something like $800 below MSRP, escaped sales tax & shipping cost (750+ lbs.) and it was sitting in the back of my truck 24 hours after I ordered it. I also bought a Miller plasma cutter from them and got the same excellent service.
[url=www.millerwelds.com/main/education]Miller[/url] has some great online articles & resources. You can also get great tips & info from [url=www.tinmantech.com]The Tin Man[/url], especially on gas welding of aluminum.
[Edited cuz I should reread before I hit "submit"]