Is there sufficient evidence to resolve this debate one way or the other? In spite of the massive amount of articles and books written about the early Christians' views on war and killing, no scholar had ever collected in one volume all the extant data (literary and archaeological) on the topic. I did that in The Early Church on Killing, published by Baker Academic. This sourcebook contains every extant statement I could find by Christian authors up to the time of Constantine relating directly to killing. It also includes all relevant inscriptions on tombstones and other archaeological data.
In an afterword, I summarize the historical record. Starting in the late 2nd century and then increasingly in the later 3rd century and the first decade of the 4th century, there is evidence that some Christians were serving in the Roman army—at least a few by AD 173, and a substantial number by the late 3rd and early 4th centuries. Unfortunately, our sources do not enable us to say how many.
On the other hand, there is not a single extant Christian author before Constantine who says killing or joining the military by Christians is ever legitimate. Whenever our extant texts mention killing—whether in abortion, capital punishment, or war—they always say Christians must not do that.