I can't answer your question about Songwriter, but I've used an earlier version of the full Finale product. IMHO the Finale implementation of tablature isn't very flexible and I certainly haven't been able to get it to produce what I need, which is lute tablature for instruments with 6+ courses. (Sibelius is better at this, but still not wonderful.)
Both Finale and Sibelius come bundled with 'lite' versions of scanning software (SmartScore and PhotoScore). Both of these are pretty good programs and about as good as each other (though I can't remember offhand if you need the full products to get multiple voices) but the results depend on the clarity of the original copy and you always need to spend time editing the output. I don't use SmartScore, but I do use PhotoScore quite a lot; the latest version is fast and works well, though I've always disliked the editing window. I don't think voice recognition is 100% with either product, though it depends a lot on how clear the voice layout is in the original - and in a lot of guitar music the answer is 'not very'.
On MIDI classical guitars, there's an information page here: <http://home.epix.net/~joelc/git_show.html
> ; some of the links are out of date, but you can Google for the products mentioned. I'm pretty sure Godin do a nylon MIDI guitar. I tried out an Axon AX-100 pitch to MIDI converter a few years ago; it used a 'hex' pickup, which was essentially a bridge saddle replacement with six individual transducers. I gave up on it as it wasn't particularly good for entering MIDI in a music program (a MIDI keyboard is easier) and the amount of delay between plucking the string and sound output was disconcerting in live performance - though I believe this has improved on more recent models.
As an amateur arranger I originally thought I would need MIDI input quite a lot, but in fact I hardly ever use it. This is because I can find versions of most of the music I want to arrange [mostly renaissance and baroque] (a) already encoded as MIDI files, (b) in existing versions which I can either scan or convert to MusicXML using PDFtoMusic Pro or (c) in lute tablature programs, convertable to notation. The only time I need to use note entry is if I am working from lute tablature originals and I guess a MIDI lute would help me here