I'd have to say MMO games because there's usually more depth in the features. There's this browser game i used to play where it had guilds but it basically only deserved as a extra friends list, there weren't guild wars or leaderboards.
This is a tough one because MMOs are great for total immersion and feeling like you are in a world, but browser games are great for relaxing to and doing something in between other tasks. I tend to burn out from MMOs because of how much you need to be involved to upkeep your characters and farm gold to upkeep your characters. Browser games don't require so much upkeep, you basically get out what you put in, and there's less pressure to upkeep your character to do the hardcore stuff.
