Fucking BADASS. You need to code up some DEMOs now XD
Hmm...C64 mode or C128 mode?
Or, hell, CP/M mode. The C128 has a 2 MHz Z80 and shipped with CP/M disks (which I have). :p
Also, "Fixed" SID?
There were two SID chips, the 6581 and the 8580. The former is the one that shipped in all C64s and early C128s. The C128D -- the reduced cost desktop unit (the one pictured) -- has the latter.
The 6581 didn't follow the specifications properly and, thus, the chip had a few bugs, particularly in the filter. The 8580 chip follows the specs and supports all of the features correctly. However, as a result of that, not all SID tunes play properly on the 8580. In general, songs tend to sound muted on the 8580 due to the differences in the filter.
There are some songs that were written for the 8580, but given the dominance of the C64 and the 6581 chip, they are a minority.
In summary, the 6581 is the most popular SID with most songs written for it, but it didn't follow the specification properly. The 8580 follows the spec to the letter, but is less popular and doesn't match the output of its predecessor.