What is in my post is only my opinion, opinions may vary
My vote is for the PC.
Mainly because for shooters and MMOs, the keyboard mouse (I feel are better at that purpose)
I hate how now systems like the xbox/ps3 are downloading, unpacking, installing updates like a computer. And require you to install software from games to get the most out of it. (optional) With that in mind, I figure I might as well play PC if I can't just pop and play.
I also love the ability to just upgrade my system to current day specs, and be able to play newest release games at good frame rate.
Hate how loud a xbox or ps3 can get compared to a PC
The systems don't allow for multi-monitor gaming (forza excluded but I recall needing 3 360s for that)
Load times on systems stink.
Its false that I can't use a high end PC while on a couch, I do it all the time when I just want to relax.
Though I do give the systems something, exclusives, ease of knowing when you buy the game its going to run, and being small form.