Grado Platinum: which is better high or lo output?

Theoretically, the shorter signal path the better is the signal. In lo-output cartridge the length of coil is smaller than in hi-output.
The only question is how low you can go with your phono i.e. Platinum lo-output version has output of .5mV which requires a minimum of 55 dB gain.

Grado Platinum isn't the end of the world; if you go for Benz-Micro Glider .8mv output version you will be OK with built-in MC-phonos. Ordering it from Hong Kong will keep your budget the same as with Grado Platinum. Very rock-friendly Dynavector 20Xl, 20XH. I assume that 20XH is more suitable for your built-in phono for the same price as Grado Platinum but overall performance is noticably audiable.
Going for the lower output than .9mV will most-likely require a good stand-alone MC phono preamp (~45dB of gain).
Low output is OK with rock and OK with any kind of music as long as you have enough gain to bring it to preamp.
Psychicanimal statement is right that the cartridge output has nothing to do with kinds of music.