Hi there,
I own both tools and if I were in your position, I would definitely choose the guitar cutter and leave the caramel cutter for another time. If you make a lot of ganache, nothing cuts like a guitar. I have a background as a savory chef and have very good knife skills. with that said, It is very difficult to cut hundreds or thousands of pieces of ganache evenly. Also, you can cut caramels with a guitar cutter. I know that there are a number of chocolatiers that say you can't, but it can be done. You must be very careful with the final temperature -- if the caramel is too firm, you will break the strings. If the caramel is too soft the caramels will stick back together after the strings go through. I find that cooking the temperature to 240-241 degrees Farenhite is a good temperature for my recipe. Recipes that a high amount of fat and a moderate amount of glucose hold their shape best when cutting in my experience.
Also, if you choose to buy a guitar I highly reccomend going with a Dedy guitar with a stainless steel base. The cheaper ones with the plastic base are not as good and they are very difficult to clean.
good luck with your decision!
Daneil