The free to play server New Horizon certainly has a large population. Many servers were merged together so now we have people from all around the world playing together. The subscription server, Smugglers Paradise, has a much smaller population, but we are slowly growing
Both servers offer a different experience. The free to play works with a cash shop model, where you can buy boosts for your character if you choose. The subscription server doesn't have these boosts. It depends on your play style which you would prefer to play. If you are happy to farm or spend real money for boosts then go f2p. If you prefer a more equal server where these boosts are not available, then maybe p2p would suit you better. Both have their advantages and disadvantages, it depends on which you prefer more