J
First, the issue of technique is too complex for my descriptive ability, but to help a bit, consider the molten weld puddle as a thermal insulator; the farther back in the puddle the less heat is transferred from the arc to the metal... this, and what is called "stick-out" (distance from contact tip- not nozzle- to workpiece) are phenomenon which can be used to modify fixed power supply parameters (eg: if you're burning through thin stock, increase stick-out a bit and/or lay back into the puddle)

Second, I believe you're using a gas mix of 75% Ar, 25% CO2... there was a typo? No big E...

Third, non-plated wire does indeed exist. There is some controversy about the Cu plating over steel electrode, but not in my mind.The Cu should lend some (pardon the pun) "element" of protection to the raw steel from rusting when stored in less than ideal environments, also perhaps it lends what may best described as lubricity to the wire surface- it's not as hard as steel and may tend to minimize wear on drive and feed components. Moreover, as a superior electrical conductor, it might provide a slightly higher arc current at identical open-circuit voltage, and it probably increases the profit margin for the entire sales distrubutiion system. On the other hand, I can quote reliable, professional opinion based on testing and survey results that this plating has NO effect on weld quality or equpment performance. (Ed Craig... a wealth of helpfull information at www.weldreality.com)
It would probably require "instruments more sensitive than a micrometer" to measure the differences, but particularly if I were planning on keeping a spool of filler wire around for a while, and all else is equal, I'd probably opt for the Cu plated material. It 'aint like we're buildin space-ships with this process.

Finally, the information you need, or at least enough of it to become functional, will also be found at weldreality.com. Give yourself a couple evenings to become familiar with all the info it holds. Pay particular atention to Ed's comments on wire alloy (as it relates to Si content... ER70 S2 vs ER70 S6, etc.) as I believe this will proove to be more appropriate point of focus than the Cu issue worrying you, particularly in multipass welds. Although an industry standard, the shielding gas you are using may not be the most versitile for you if and when you decide to make a purchase... if the welder you purchase can provide enough power to realize the benefits of a 2 part mix with less than 20% CO2 you'll appreciate it in the long run. Ed's not hot-air when it comes to gas expertise.

Regards & good luck
d

Welder's Handbook is a good welding overview, but you are correct that MIG requires a lot more thought than it gives. I get the feeling that he expects everyone to start with oxy/acetylene, then progress to other processes. Kind of leaves this feeling that something's missing.

A couple other books that might help are:

Welding Essentials: This one is more of a reference, but it has a lot more details about the effects of different settings/technique on weld quality. Like WH, it also covers other processes, but it is more detailed.

The best MIG book I've seen, though, is the GMAW book on www.millerwelds.com . Has good photos, and is more thorough than any of the others I've read. I got mine as part of their student package, which also includes a good GTAW book and a bunch of small pamphlets on electronics and other welding processes.

I wouldn't worry too much about the copper coated wire. Not a big deal while learning.

As hard as it is, try to find a position to weld so that you can see the puddle. The MIG nozzle will get in the way, but that's really important. The wire should feed into the leading edge of the puddle. I learned MIG (self taught) first, and didn't really understand it, although my welds got prettier and did well in my break tests. But once I started learning OAW, it just made more sense, and it seems a lot easier to get consistent results.

Check your local community colleges for welding classes. They're well worth the time and money.