A tough one, really. I haven't played SSBB (I have, unfortunately, never owned a Nintendo console; also, I live in Europe too) but I've played all non-handheld MGS games.
From what I've understood, you can enjoy MGS4 even without playing the previous games. However, you'll definitely enjoy it even more if you've played the previous games. Also, if you can appreciate a good story (and preferably cutscenes as well), you'll like MGS. And MGS probably has more depth than SSBB. You should also remember it's not a shooter (well, apparently you can play MGS4 as a shooter though). Oh, if you don't like Nintendo's 'Europe must wait' policy you'll definitely want to pick up MGS4 first.
It really depends. What kind of a person you are when it comes to games? What kinds of games do you like (most importantly, do you enjoy games that have even a little depth)?
EDIT: MGS will also have replay value if you want to collect stuff, a stealth device for example. Personally I enjoy getting all the stuff I can get. On top of that, MGS4 apparently has even more gameplay than the previous ones so even the first time should take longer.
