The M12 made over 1000 HP using just 1500cc of displacement. They should totally have put that in the new M3/4 and call it a day. Makes total sense, right? Except for the fact that when you read interviews from anyone who drove the cars powered by them, they'll tell you how difficult it was to manage the power band.
"the engine is going from 450 HP to 800 HP in about 1,000 RPM" [1]
So you go from a tuned STi to a supercharged big block in about 1,000 RPM. Total pussy cat, am I right?
Yes, tiny, turbocharged engines save weight, but they come at a price. It's a continuum. On one end, you've got a motor like the LT4 in the new Corvette Stingray. You can take 2nd gear corners in 3rd gear when you've got an LT4 under the hood. You'll pay a small penalty, but you can quickly recover. On the other end of the spectrum, you have insane combinations of small-displacement, high-revs, and monster turbos like the M12. Everywhere in between lies engines like the N55, S55, and the M133. Everyone's preference lies somewhere on that continuum as well, and they're all valid.
The M12 was absolutely incredible. The M133 is absolutely incredible. The N55 is a damn nice engine. IMO, the M133 is a higher technical achievement, but it's a different engine with a different character. If I were choosing between the M133 and the S55... man, that's a much tougher choice. I'd have to look at the whole picture. What car is it in? What's the design goal of the car? Is it more of a GT, or more of a sports car? It's not a simple matter of walking over spec lists asking "is X > Y?"
1:
http://www.gurneyflap.com/bmwturbof1engine.html