People who get bogged down in arguments comparing the supposed "accuracy" of the two seem to me to be missing the point, by which I mean no disrespect to you as one who is new to the calendar controversy. "More useful" depends not necessarily on "accuracy" but on what you want out of a calendar generally.
Quite simply, many Orthodox (myself included) reject the Revised Julian Calendar because it's not the calendar that was passed down to us from Roman/patristic times. In a liturgical context, connection to our forefathers through continuity in worship is far more important than mere astronomical accuracy. It unifies us not only today, but through the ages, aided by its
cyclical nature which repeats every 532 years and is completely lacking in the new calendar. There are other differences which are often highlighted (and have been in this thread) and I don't want to understate their importance, but that continuity alone would suffice as a reason for traditionally-minded Orthodox Christians even absent the rest.
There's also the issue of how it came about, which has nothing to do with the new calendar
per se but is still important to understand the resistance to it because it ushered in a period of often-bloody persecution against pious Orthodox clergy and laity whose only "crime" was continuing to practice the unaltered faith as it was passed down to them. The RJC was forced on the whole Church unilaterally by the Ecumenical Patriarchate under the highly-controversial Patriarch and known Freemason named Meletios, an act which was regarded by most jurisdictions as an empty papal pretension and ignored because even a perfectly good new calendar would need to be implemented by a pan-Orthodox council. He was chased out of Constantinople and made to resign after a fairly short tenure, but the division he caused was there to stay.
You'll often see anathemas against the Gregorian Calendar applied to the RJC, and that's because RJC was specifically devised in order to align much of our liturgical year with the GC and thus promote (or at the very least intentionally reopen the possibility of) concelebration with Roman Catholics and Protestants. That's where the problem of ecumenism comes in, which I won't go into any more than that because it'd add several paragraphs to what I'd hoped would be a short answer. Such hopes are always dashed quickly when talking about the calendar.