Firstly, I havent bothered trying out RoM, but I'll try not to be too biased...
Which ones better to play? Dunno, haven't tried RoM yet...same goes for you question about which is more fun.
Which has the more orginal concept? Atlantica. You dont have to play RoM to see that it's a WoW clone. There's no other game like Atlantica. Turn based games are rare to start with, but then Atlantica has a indepth merc system, and many other systems you wont find in other games.
Which has a more F2P friendly cash shop? Well, from RoM players, i've heard it was very cash shop based. In terms of Atlantica, I can say it's not cash shop dependant at all, and all the items in the cash shop can be bought for gold in the market. Either way, these items arent vital to do well in PvP. It's more focused on crafting good gear, and your strategy and Merc combination.
Which has the more mature community? Dunno, but in my time in Atlantica, I met a total of 1 immature person...which to me is outstanding. You need a calm logical head to survive past level 60 in Atlantica, it's not one for 14 year old brats.





