I think there is a better chance of getting a 6581 Rev.4 on a 128 than on a c64, unless its a C64C. The C64c has a better chance of coming with an 8580 too!

I loved CP/M, I remember booting it frequently just to get that terminal/DOS feeling

Bot natively, but it can:
http://www.devili.iki.fi/Computers/Commodore/C64/CPM/
(albeit shittily, since the cart was horible ;D)

I would get a C128 simply because it's such a Frankenstein's Computer. Does the C64 run CP/M? That's right.

Yep, when i had a C64 i used a modded 128 power supply and it worked like a charm.

The PSU on the 128 is known to be more reliable than that of the C64.

Some people use connector-modified 128 PSUs on C64s because of this.

C128 is cool. The babes prefer C64 though. It seems like the SID-chips of the C128 often get semi-destroyed and make amazing noises though. So, babes or noise. It's your call.

You can't use anything extra on the C128 (only extras are memory and a new VIC and the VDC for 80 column display, oh and a lame 2Mhz mode) for music making, and not in C64 mode. So it doesn't make much of a difference. It has some compatibility problems, just very few. And it's far bigger.

Some of them have an 8580 though, but most have a 6581.

I'd get a C64 unless it's an 8580 C128.

I have the opportunity to pick up a Commodore 64 or 128.  I was wondering if the extra processing power of the 128 matters in making music, especially when booted up in 64 mode.  I have an unused MSSIAH that I am super excited to start using, I just want to pick my tools wisely.