Well, quite obviously, this is the Monomachine to get:
http://www.elektronauts.com/t/fs-us-nothing/1291/8555/page:1#8555
It’s my SFX-6 of course!
To answer the initial question:
If you can find the MKI (SFX-60) for a good price and you’re debating whether to go with a MKII, the main addition is the User Wave machine. It is a fun and useful synthesizer voice.
The noise of a MKI will not bother you, especially in the context of a full composition.
If you’re in the market for a SFX-6, go for it, especially for the performance aspects. Nothing is as fun as playing on the attached keyboard in poly mode, using the joystick to modulate four distinct parameters. The keyboard is great for playing arp patterns with notes more than an octave apart.