In the U.S., we do not respect the requirements of Italian DOC or DOCG, thus, there are a lot of balsamics that are sold that are not Balsamic, not from Italy and even not at all from wine grapes. The reality is that the stamp is the only proof that what you are using is balsamic vinegar at all. Now, that being said...
All balsamics are aged, and any one that has the stamp is going to meet some basic minimum requirements and it will be pricey. This does not mean that it is the only game in town, there are excellent aged vinegars from France, Spain, Sweden and the U.S. (which I use normally) which are all excellent.
I would tend to disagree with the use of a more aged vinegar with a mild acidity for what you are doing Tish. I believe the acidity of a younger Balsamic, or similar aged vinegar is what you would be looking for. The slightly sour profile will bring out the sweet and salt of the melon and ham.
I actually use O California Balsamic, for the reason that is has up to 35 times less detectable lead in it. Plus, it is local, cheap and works just fine for me.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."