I think the HardSID cards win in the pure SID coolness category, but the SID support in the Catweasel gets the emulation nod, as it supports paddles on the joystick ports. Actually, joystick ports and support for paddles as MIDI controllers would be a nice feature for the HardSID cards. I'm getting off topic.
I've never put a SID chip in my MK3, but I hear it works great. But even with two SID chips, the MK4 appears to be designed for the emulation enthusiast rather than the musician.
Trev