I have a LencoSpot and wouldn't be without it. That said, I definitely think a Miller spotwelder would be very useful in many instances.
The problem with the Lencospot is that, at least on my machine, the leads are very heavy, short, and somewhat stiff. It takes a lot of effort and some amount of creativity to use in confined areas like when spot-welding a decklid filler panel. Also, you can get burn through pretty easily if you are accidentally touching metal that you don't want to be spot welded. Also, it takes up much more room in the shop than the miller.
Advantages to the Lencospot are that you can weld areas that you could never reach with the tong-style welders.
Also, you don't have to pinch the metal between the contacts with a Lencospot. If the two pieces of metal to be welded are clean and have good contact, you can make two spot welds at once by pressing the two leads (contacts) against the sheetmetal and pushing the button. For instance, if you were welding a shaker hood together, you could weld the underhood brace to the bottom of the hood using this method.
You can use a Lencospot almost anywhere you can use a miller, but the reverse is not true. The flip side is that where you CAN use a miller, it would be quicker and easier to use than a Lencospot.
So the short answer is that in a perfect world, BOTH tools would be in your inventory. If you can only buy one, I would suggest thinking about what you would MOSTLY be using it for and then choosing the best tool for the job.
