How hard do you find this game compared to chess?

Once you've learned the Chinese symbols it's about the same difficulty as chess. However, the cannon adds an extra layer of complexity.

On the other hand, the extremely limited movements of the King/General, the elephants, and the guards, make the game less complex.  It's still quite fun.

Japanese chess, Shogi, along with it's many size variants, is quite different and more complex.  You can use your enemy's captured pieces, brainwash them, and then re-enter them on your board to use against him.

Is there any way to play these two chess variants online, like with www.chess.com?

clubxiangqi.com is pretty good for playing Xiangqi. You play with a time limit similar to live chess here.  The standard of play is pretty high.

Incidentally computer programmers consider Xiangqi to do be more complex than chess.

I'm about to learn how to play Shogi. It looks very interesting indeed.