Unfortunately in canon, there is no straight answer, but I'd guess that the vast majority of people would conclude that the Enterprise-C variant came first, mainly because of the bussard collector caps. However, I've noticed that the Yamaguchi's sensor dome underneath the saucer is more reminiscent of the Excelsior's dome rather than the more simplistic Ent-C dome, so YMMV. Plus, the only Ambassador bridge interior we saw was the Ent-C's, and it was reminiscent of the TMP-era ships.
One of the things that irked me about the Ambassador class is that we know so little about it. We don't know when the prototype was constructed, we don't know how long the class was in production, we don't know why most of the known ships of the class had registry numbers of 1XXXX and 2XXXX while the older Excelsior and Miranda classes had registries of 3XXXX and 4XXXX, and we don't know why we never saw the class again when we saw those huge fleet shots in DS9.