They are the same technology, as far as i know the exact same machine. Either will perform similarly, i think the thunderbolt is slightly more adjustable. However this is less important than you think, rod angle and manipulation is more important. When welding pipeline, you have a remote in the ditch with you, which your helper adjusts on the fly to help react to conditions as you go. While this would seem necessary, it's not, it's for speed. On building trade jobs, you string out a few hundred feet of lead, ground the machine to the closest steel i beam, and set your heat on about what you think you need. Then you go and weld with it set at that heat all day, with different rods, thicknesses, and positions. You use rod angle, arc length, and other tricks to achieve the same thing. If you need more heat, you angle the rod slightly towards the direction of travel, if you need less you angle it back towards the puddle. A tight arc will increase the amperage, but will lower the voltage, leading to less heat spread out. This will allow more control, because metal flows to where the heat is. Really, that simple fact is welding in a nutshell.
Miller and Lincoln are the two main welding manufacturers anymore. Esab is up and coming, arcon is the old powcon, airco have disappeared (Miller bought then?), and Hobart is now Miller (They used to be a very viable alternative). Miller is usually seen in shop settings and cheap engine drives. They are lacking in arc quality and stability compared to comparable Lincolns, although they are closing the gap. Miller is known to push the envelope of technology, sometimes before they have perfected what they are trying to do. Lincoln seems to get it perfect before they release it, which means Miller will introduce something and then Lincoln will improve it. If my prejudices haven't made it clear, pipe welders are very particular
Most welders i know will take a Lincoln anything over a Miller, but will do xray work with whatever you give them. In the end they are both reliable machines, kinda a ford Chevy thing (until you start talking downhill machines). In pipeline (downhill) until quite recently if you showed up with a Miller the inspector wouldn't even let you test. Even now there are only 1 or 2 Miller machines they will allow on most ROWs.
Basically either machine will serve you well bud. If i had a choice, i would go with the Lincoln.
