Both are in D tuning, the Mel Bay one being a bit more basic (how to tune, notes on the fretboard, first position chords, five simple songs, some strokes/strums, moveable chord shapes for all major and minor chords, basic major scales, and some cool beginnings and endings) and the 'New Original' one having a few more chord melody pieces. They both aim at the beginner, have 48 pages and there is quite some overlap. The Mel Bay one claims to be a guide to chord melody playing in its subtitle, but that's a bit far-fetched - although the chord inversions do set you off on the road.
Smeck never published an advanced method book to my knowledge, but he did have some fine songbooks and books with instrumentals which are more instructive to advanced players. Although they are not published anymore, they can be found on ebay or even easier as downloadable pdfs. Also all in D tuning.
The difference in tuning isn't that mind bending. If you just 'play the dots', you'll do fine in C6 tuning. It's only when playing along with someone else that you'll either have to transpose or tune up to D6. Both books are crammed with information, but they're introductions: if you've been playing for a few years, most of the contents will already be familiar to you.