I thought I give an update on this. I bought a smallish Korg CA-2 chromatic tuner and the CM-200 contact microphone. The tuner is pretty light weight and smaller than my iPhone. I compare to a typical pack of guitar strings:
I figured from pictures I found on the web and manuals, that the higher specified models (CA-40, GA-40, TM-50) do not offer a better read-out accuracy than the smaller and lighter CA-2 and GA-2. To tune a classical guitar, I don't need an output to an amp and I personally prefer a mechanical clockwork metronome with a swinging arm, so the 40/50 models were not of strong interest. I'd quite like the fold out stand of these - there is nothing on the rear of the CA-2 to make it tilt up. Though in the end with me the smaller size wins. Specification of the tuner detection accuracy is the same for all of these: +/- 1 cent.
I like the design and colour scheme of the GA-2 (guitar version) a lot better then the white/yellow colour scheme of the CA-2 (general chromatic tuner). Though I figured that the GA-2 is a lot more limited. It can only tune open strings, checking pitch for fretted strings is not naturally supported. For strings 2 to 6 one can sort of do this using the "flat" feature, if you think a bit, but it isn't meant for this and the usability would be inferior to a chromatic tuner. The read out of the GA-2 scale is by a factor 1.6x less accurate (steps of 1 Hz for the concert pitch, approx 4 cent) than the CA model (steps of 2.5 cent). Apart from Peter's answer above I haven't seen any statement, regarding whether or not the GA tuners make positive use of the information: "Hey I am tuning a guitar". Peter thought there is not much in it.
Having used it for a few days to tune a student quality instrument with D'Addario EXP46 (60h to 80h of action) I'd like to share the following comments:
- Overall I am quite happy with the tuning I get. I think better than the iPhone apps I had before.
- Damping the guitar between strings to stop sympathetic ringing is advisable
- Sometimes it misidentifies the tone, e.g. showing a B-flat instead of an E on the first string. Changing the right hand position, stopping sympathetic ringing (dampen the guitar) and/or moving closer to the tuner typically solves this.
- Compared to Guitar Tuna, which is the "sharpest" app I have on my iPhone 6s, the Korg is about 4 cent (1 Hz regarding concert pitch sharper). However I have apps on my iPhone that are by a similar amount flatter than Guitar Tuna. So this seems a typical level of accuracy, how a given signal is translated into pitch. Please note, I have been using Guitar Tuna as a reference here, but do not want to say it is more accurate than any of the others. To me it shows, that when doing e.g. an ensemble, one should use a common tuner/tuner app.
- I find the accuracy of the readout reasonable, not to fuzzy and still accurate.
- You are risking your fingernails when opening the battery cover (2x AAA-batteries). Since Korg claims a battery live of 100 to 200 hours, depending on battery quality, you shouldn't need to open it very often.
- The CM-200 contact microphone is quite large, when compared to the tuner. The signal seems to improve and the tuner seems slightly more responsive when using this. It opens wide enough to fit the headstock of my guitar, which the D'Addario micro NS doesn't. I am currently not sure I would buy the CM-200 again.
I hope someone here, finds the information useful.