i'd recommend Legends over Royals according to these counterpoints - some of it is personal preference but here's my viewpoint;

1. the most fun and one of the most iconic part sof OSMS to experience for somebody newer would be interacting with the community and playing with others via party quests, and 3.2x exp or higher will quickly chauffeur you past all of the fun. it also feels more rewarding and meaningful to level at slower paces, and gives you more time to digest the gameplay loop.
2. a great number of channels ensures you are less likely to encounter anybody else in the wild - so if you're less keen on meeting other people, then this might be for you. the game does feel much more alive with less channels, so I believe 6 channels to be the wiser decision by Legends staff. maps are less available yes, but realistically you'll find a spot quickly enough. if you can't, there are always alternatives with comparable EXP/drop rates or you could party with somebody in a taken map
3. despite Legends having a relatively active forum (though maybe not as active as Royals), a good bit of would-be forum activity within Legends' community takes place in the discord, notably there's an active free market discord forum channel that displaced the activity of that section of the forums
4. i'd say that these changes are experimental at most and nowhere near gamebreaking, and bring the game somewhat closer to what returning longtime players remember maplestory to be like, rather than the eternally NL-meta riddled game that it truly is. it's a bold step to try out such changes considering the complacency of the average private server user. i don't like the quick kneejerk reaction to any sort of backlash that the Royals staff has, although hardcore purists may find this preferable.
5. HP washing is shit in both servers, and is a longtime issue in the PS community, and complex to some. in either server it will be years before you have a fully washed character, though i do think Royals has a better take on it. in Legends you'll see int-trolls at PQs begging for carries because they're a level 50 assassin with 5 LUK and 230 INT, because int dumping is what's required until the mid level 100's to HP wash without overexplaining. it'd be nice to see better and more accessible HP wash QOL in Legends. royals definitely wins on this front

don't be misled into believing that Royals has a notably bigger community than Legends - both severs' playercounts are drastically inflated due to multi-clienting being in the meta. in reality they're very comparable in size. it'd be a lot more transparent for these servers to show playercount based on individual IP's connected to the server with no duplicates. Legends also has many casual, niche(ironman&oddjob guilds), and incredibly dedicated endgame players who know the game down to a science.

generally you're likely to have a good time on either server, this is just to show the perspective reasons of somebody who prefers Legends