Personally, i've only played enough to experience gameplay on both games. Can't say much about community, although RoM does have quite a big and friendly population during the times I played.

It all depends on your style of play, Atlantica has turn based combat instead of real time like RoM. I got a little bit irritated at Atlantica because of that, but that's just my personal take on it. I prefer playing my character alone without any mercenaries.

RoM just got a new major update as well, but that doesn't mean Atlantica isn't introducing new classes for your party. They're both good games in their own manner. Although.. Atlantica has a stamina bar which when drains to 0 doesn't allow/greatly reduces any exp you gain.

For the old MMORPG generic feel, go for RoM. If you want to try something different, go for Atlantica. Either way, I hope you do enjoy yourself, because that's the whole point.