Since I'm apparently a "major gamer," I guess I should throw my hat into the ring. Not sure if I should feel happy about this title or not
Anyways, first things first, Assassin's Creed is multi-platform, it is available for the Xbox 360, PS3, and PC. There are also other titles, not sure if all major titles of the series (1, 2, Brotherhood, etc) are available on portables like the PSP or DS, but I believe they are (don't quote me on that.)
Down to your question Arcanine on what console you should get, read into the games you like to play. You noted that you're playing Mass Effect and want to play that. You can technically play that on both consoles, however, Mass Effect 1 is only available on Xbox and PC. Mass Effect 2 includes those as well as the PS3. The PS3 version of ME2 let's you start with a virtual comic that basically tells the events of the first game and let's you make basic decisions so you can create a tailor made story for you, but it's not as in depth as actually playing the game and importing the save file.
So, my opinion, at least for that game, would be to get an Xbox. However, it's really up to you. Do some research, as in, look up the games you want to play and what platforms they are available to play on. As Sen said, there's no major differences between consoles other than Sony's lack of security and the Xbox's ability to play the now obsolete HD DVD (woo Blu-Ray!) Otherwise, it's not that big of a gap between them. When I'm not so hungry, I might list out the differences between consoles
Also, for future reference, I only have a Wii, PS3, and a PC. I do not own an Xbox, nor do I think I will since I can pretty much play all those games on PC, though that could change.
Anyways, hope that clears things up for you a bit Arc. If you guys have any questions for your apparently "major gamer," feel free to contact me through PM on here, or better yet, if you have Skype, add me (Necros1215) and you can reach me there basically all day. Note: This goes beyond gaming advice, I've always wanted to be the big brother figure in this clan, so if you need me for anything (except plumbing

) feel free to contact me.
Edit: Got some food in me and found this link, hope this shows some differences between the consoles to consider:
http://www.consolesandvideogames.com/xb ... -consoles/