Game programming is indeed, very difficult. Jagex created RuneScape over a series of years. Though Java would be "easier" for what you're trying to make, you would still find it quite difficult. My best bet is that Andrew/Paul Gower had been programming in Java for many years before they even started to make RuneScape, so if you want to make something in this context then you're better off actually learning the thing first, programming second. Remembering to keep a balance in your life. There isn't much point living if all you're going to do is live for making this game. Andrew and Paul Gower for example, they started RuneScape as a project in their parents home I believe, but once it got popular, they started to hire people, and more and more staff, and now they have millions of subscribers, and they're millionairres.
Keep a balance, but you'd have to have been learning a language for a couple of years at least before you start to tackle game programming - even when you consider RuneScape. Plus... Game programming is generally done in a team. Hence you'd be better off first joining a modding team for a game like Half-Life first, to get a little hands-on experience.
Both languages suit what they're made for. Depending on what you wanted to do, you could make this sort of game in either of them - though C++ would be the - ah - "generic" choice. Meaning, most people would choose C++ for obvious reasons. Java is still good, though, just you couldn't fine-tune it anywhere near as much as in C/C++. Its really a hard decision. Seeing as you'll be wanting to learn something easy to start off with, I voted Java.
I hope my extremely long reply that took 5 minutes to write really quickly with my mind running at 200kmph really helped you a little bit
and of course, this has been debated in short like 10 times already, I just want to feel like I've actually posted something useful ^_^