At the moment, I'm going through a phase of the latter.
Granted, I've now completed more of Deus Ex than I ever have before, but I won't consider myself a monogymous gamer until it's completed.
As I've had a week off from work (spring break), I've been able to dabble in a whole bunch of other games too.
I'm still playing Team Fortress 2 (PC) a few nights a week, and now I've even roped my friend Dan McCue into playing online with me.
I've gone through one round of Lost Planet: Extreme Conditions on my Xbox 360. This was the game that first got my warmed up to the idea of buying a console again. It hasn't held up well over the years, but it's still ok.
Asia online, aka Exteel is a free-to-play, microtransaction-based, PvP mech game. It's free, and worth every penny. It should be a market hit in China, where the piracy to retail ratio is currently around 6,000:1. Although, if you're reading this now, there are probably about 100,000 people who are better than you.
I jumped back into City of Heroes for an hour or so this week too. It's the only MMO that I still pay a monthly fee for. I keep forgetting how awesome that game is with a good group. I leveled up my character in under an hour, and had a great time.
I was working on Diablo II (never played) for the past week or so, but decided to take a break from it in order to go all ADD as described above.
Phew!