WAIT!! I think the only person to respond that actually owns both games and sunk in close to 10 hours on both.
I like the Naruto game I bought it the day it came out I was excited about it because I love naruto. But honestly at about 10 hours I'm getting a little bored. They trid a little to much with it, oddly it's a fairly polished game and running around Konoha is great, but when you run off on missions and particularly play platform type gaming parts, the game kind of gets annoying, it's the same puzzles over and over again and the controls while doing these parts gets annoying. In Konoha after a while there is not alot to do after a while, although it is very satisfying for a while, escpecially as a fan. So this is a break down, it's sand-box elements are fun but limited, it's fighting element looks great, plays great, but I found you end up repeating the same technics over and over battles are often to fast to get off the cool moves. And the platforming elements are mediocre, although if your a big fan I don't think you'll regret getting it, but if it comes down to this and Mass Effect, I'd get Mass Effect.
Mass Effect while there is a few weird glitches it doesn't stop me from saying at least for the short amount of time I spent on it it's the best game I've ever played. My only warning is it's probably not for gamers who just want a halo type experience, while much of the slow parts can be skipped, the game thrives on intense conversation with varies dialogue option that are kind of a throw back to adventure games, actually it kind of reminds me of playing Shenmue, with that said it should be known that ME's slow parts are amazing and better then any other games slow parts, and that's not to say their isn't plenty of action, and all kinds of it and the depth of options you have, from where to go, what to explore who you'll bring with you, what guns and what attachments, and all the extra skills and such will give you quite a bit of meat for any rpg fan.
So read that and make up your mind. I was quite excited about both titles, but Mass Effect is the only one that lived up to the hype, while Naruto is not bad by any means it does fall short of what I wanted it to be.