[sinistral]

Even with light strings on it, the HD-28 will feel a bit different from the 000-18, because the scale lengths are different. The HD-28 has a standard scale length of 25.4”, and the 000 has a short scale length of 24.9”. A half inch doesn’t sound like much, but it is noticeable (to most people). A 000 body size with a standard scale length neck is called an Orchestra Model, or OM, and it typically strung with light strings. That would be the closer comparison. As Bob notes, if you put lights on the HD-28, the action will likely lower slightly because the light strings have less string tension than medium strings.

Martin has a long-standing reputation for shipping their guitars with high action, but in recent years they have made strides to set up their guitars with more “normal” action. Taylors on the other hand are known for their playability “out of the box”—sometimes referred to pejoratively as acoustic guitars for electric guitar players. Ironically, the action on my Taylor 614ce is slightly higher that several of my Martins “out of the box.”

There are a few simple things that you do to check if your guitar is poorly set up. For example, if you put a capo on at the first fret, is it much easier to make chords in the first position? If so, it’s likely that one or more (or all) of the nut slots are high. I find that it’s easier to measure action at the 12th fret in millimeters. Action is measured from the top of the fret to the bottom of the string. Since I’m not a heavy strummer, I prefer action on the lower side—2.4mm - 2.5mm or so for the low E and 1.75mm or so on the high E side. I find that, if the action is too high, it’s harder to make chords higher up the neck, and, when bending strings, the adjacent string hits my finger at an uncomfortable height.

Ideally, the truss rod of your HD-28 was adjusted properly for the string tension at the factory so you shouldn’t have to adjust the relief (slight bowing) of the neck. A simple way to check the relief is to press down the E string at the first fret (or use a capo at the first fret) and press down the string at the 14th fret. There should be a small gap at the 6th or 7th fret (.10mm .25mm). If there’s a big gap, the neck has too much relief, and if the neck is flat (or worse, back bowed) it has too little relief. If the relief isn’t correct, you should definitely have a tech go over the guitar.

A completely unscientific way to get a feel for action is to go to your guitar store and play several Taylor guitars. Take one or both of your guitars with you for comparison. Don’t worry about measurements, just focus on feel. Play chords in different positions up and down the neck. Bend notes (if you play any songs in which you bend notes). Ideally, you should find the 000-18 the easiest to play given that it has light strings and a short scale (Taylors typically have light strings and a standard scale). The medium strings on the HD-28 will have the “heaviest” feel. If you switch the strings to lights on the HD-28, that would be more of an apples-to-apples comparison.

Hope that helps.