Should I learn rhythm first, before starting to learn lead?
No. The fact is, you donít improve in neat little bundles: "Now I have this area mastered, so I can move on to the next one and master it." Improvement is holistic and overlapping. Each skill overlaps into the next. And those two divisions, "lead" and "rhythm" guitar, are really just two sides of the same coin -- it's all guitar playing. So you can and should learn both together.
But isnít playing lead harder than playing rhythm? Generally, yes. That's why people will tend to focus on rhythm first, then later progress on to soloing. But is that always true? Is lead always harder than rhythm? No. I could fish up a few rhythm parts that would eat your average lead solo for lunch and not think twice about it. And I could also find a few very easy solos. So letís not draw a big rule about these categories in terms of what we can or cannot learn.
What is important is that you tackle music that is roughly at, or just slightly above, your present level. (No Paganini on the first day.) Now in the method books, I have already taken care of all that for you. All you have to do is take it one page after another, and the level of difficulty slowly rises, to continually challenge you at the right level. So go through the Metal Rhythm Guitar and Metal Lead Guitar methods at the same time. Now when it comes to learning songs which are not "graded" by level of difficulty, itís probably true that youíll be learning mostly rhythm parts first. But look for easy solos to learn here and there as well.
I should say one more thing about this whole rhythm/lead thing. In the end, itís actually rhythm guitar that turns out to be more critically important and more difficult: the subtle qualities of groove and so forth. I mean, the typical pattern is that someone starts out learning rhythm guitar because it seems "easier," then they work on playing lead because itís faster and therefore "harder," and then -- if they get far enough -- they eventually realize that they missed a few things on the whole rhythm thing and they go back to really learn how to play rhythm well. So maybe lead isnít harder than rhythm at all. Each just requires slightly different skills and techniques (which also overlap a lot).
So if youíre an absolute beginner, start with Metal Rhythm Guitar Volume One and Metal Lead Guitar Primer. If youíre an intermediate player, start with Metal Rhythm Guitar Volume One and Metal Lead Guitar Volume One.Working out of both together also has the benefit of giving you a lot more variety in your practice, so you don't get so tired of practicing the same stuff.
You can and should learn rhythm guitar and lead guitar simultaneously. Go through the Rhythm and Lead Guitar methods together, at the same time!