Well, until I know the bigger one is going to work, I'll hang on to it. The M90 is made for much bigger engines, so it may be overkill. The reasons I want to try it are;
It should take (theoretically) less RPM to move a given amount of air, which means less heat buildup,
Turning slower = quieter, as the M62 was pretty loud,
Gives me some headroom for future boost increases, whereas the M62 wouldn't have gone much further, and;
Repair parts (drive couplings, bearings, seals, and different pulleys) are widely available for the M90. This Mercedes-app M62 has an odd input shaft/bearing arrangement, parts likely must come from Benz, such as shims, pulley bearing, and seals. And because of its odd bearing, the pulley that's on it is the smallest she'll take. The pulley rides on a sealed roller (or perhaps double-ball) bearing.