Fuck, Skyrim is a long boring, repetitive and simple RPG with barely any story or personality to characters.
Mass Effect is a well lengthed immersive, fairly simple RPG, but the choices you make and the characters you meet all feel like they have a purpose. The combat is actually fun in Mass Effect and you might want to play through it more than once to try a different class or make different decisions to see the outcome in the next game.
If you want something that will hold you in and want you to play the next game to continue with your character that you'll have some attatchment to all while enjoying fun combat, get Mass Effect.
If you want something that will allow you to do the same thing for hours, let you feel like you're not apart of the world, give you a story that you won't give a shit about and will end in the most boring way possible all while giving you boring combat against the same one or two enemies, get Skyrim.
You might like Skyrim at first, but that thing slowly became a piece of shit in my eyes. Especially with everyone saying it's the best game of the year / best game of all time. I can't even see how best game of November 2011 is even close to possible.