Of course Miller isn't the only company making welders, and companies such as Lincoln Electric and HTP America offer MIG and TIG welders. I'd suggest doing a little research and making a decision on what's right for you and what you'll be working on, or may want to work on in the future. All the welder suppliers have websites that are informative and can guide you through any questions, with Miller's being particularly helpful. In fact, if you visit the "Order Literature" tab at www.millerwelds.com, you can download a TIG Buyer's Guide.
One thing we haven't touched on so far is what size welder you might require. The small portable MIG welders are very handy and easy to move around, but are they up to the task of chassis fabrication for instance? They are classified by how much amperage they can generate at a given duty cycle, which is determined as the number of minutes the welder can operate in a ten minute cycle before it needs to cool. Thicker metals require more amperage than thinner gauges too, so while a 130 or 150 amp MIG welder with a 30 percent duty cycle may suffice for small shop jobs up to 3/16-inch thick, you may be better opting for a 200 or 250 amp MIG with a 40 to 60 percent duty cycle if you plan on tackling heavier fabrication. If you're going to spend the money, why not just spend it once?
So which to choose? MIG is easier to learn, is capable of welding light gauge or thick material, and will weld all common metals, from mild steel to stainless and even aluminum if you change gas and use Miller's spool gun (assuming you have a new Miller welder!), and is faster. TIG offers precise welding on thin material with less distortion than MIG, produced high quality, smaller, and good looking welds, can weld steel and aluminum with Argon, without swapping bottles, and offers adjustable amperage control while welding. You pays yer money and takes yer choice, though if it were my preference, I'd opt for the TIG, even though a MIG is perfectly capable of chassis work, but then I already have a MIG, so I would say that wouldn't I?!