Major 6 vs minor 7

[gtpiano]

I notice that in jazz the 1 chord is sometimes replaced by the 1 6th, e.g., C replaced by C6. In popular music, the C is often replaced by the Am7. Of course, the 1 6th chord has the same notes as the 6min7th of the same scale but the bass note changes the sound. Why is this suggested differently for jazz than for popular music. I am guessing that if the Am7 is used in popular music (in the key of C major) the bass player would be better placed to play a C note over that chord. However, the bass player would probably not do that intuitively. Peoples thoughts, please.

[j_tour]

Yeah, it's true that, especially in older "sheet music" type publications, you'll see the added 6th chord in the chord symbols.  Instead of, just the generic maj7 chord.  I really have never decided what to make of that:  for all I know, it could mean sometimes a first inversion chord.  I treat that kind of notation as just kind of optional, vanilla.  I think everyone just plays it as a maj7 chord, unless maybe doing "trad jazz" or something.

I should note that adding the major sixth to a minor chord, both in voicings or in creating melodies is extremely common, especially in minor blues, but elsewhere as well.  I don't believe I've ever seen that notated in chord symbols, though.

About creating basslines on the third of the minor chord built on the sixth degree of the major scale? 

I can't say I've noticed any deep tendency to do that, as though it were a kind of a rule.  It's just an option, and it has the potential to set up a nice pedal tone.  (As in, for example, Bill Evans's "Peace Piece," which is itself drawn from a fragment of his arrangement of the standard "Some Other Time").  I'd associate it with some kinds of modal music, I guess.

One caution might be that the harmony instruments (guitar, piano, lets say) might very well play that I-vi and substitute a VI7#9b9 or something else, like a bIII7#11, on the spur of the moment, which might not be ideal with the I note on the bass.

I don't know, just personal taste, and where the melody of the bassline itself takes one.