Which is better for making music from scratch?

The Roland S-1. It has oscillators, a filter, effects and a sequencer, so it creates sound on its own. The M6 USB only passes audio through from other gear.

Can I use the S-1 with the M6 USB together?

Yes. Run the S-1's stereo output into a line channel on the M6 USB to blend synth parts with decks or mics. That pairing covers both sound design and mixing.

Which is better for a live DJ set?

The Numark M6 USB. It has a crossfader, channel faders, phono inputs for turntables and XLR outputs to a PA. The S-1 has no inputs and is not a mixer.

Does the S-1 last as long as the Numark?

The M6 USB is built heavier for stage use and should take more physical abuse. The S-1 is a compact plastic unit; treat it as a desktop and travel instrument.

Is the S-1 worth stepping up to over a basic synth?

If you want SH-101-style tone plus wave drawing and chop features in a tiny box, yes. If you need more than four voices or full-size keys, look higher in Roland's range.

Roland S-1 Tweak Synth vs Numark M6 USB

These are different tools, not rivals. The Roland S-1 is a pocket polysynth for making sounds and patterns; the Numark M6 USB is a four-channel DJ mixer for blending sources. Buy the S-1 to compose and design tones, the M6 USB to run a DJ or live rig. The S-1 wins on creative depth and portability; the M6 USB wins on inputs, routing and build for stage use.

Choose the Roland S-1 if you want a small synth to write basslines, leads and patterns and to shape your own waveforms.
Choose the Numark M6 USB if you need to mix turntables, mics and line sources with a crossfader and XLR outputs.

Prices

Roland S-1 Tweak Synth: $229.99 at Amazon (last seen Nov 9)
Numark M6 USB: $249.00 at Walmart (last seen Nov 9)
Difference: the Roland S-1 Tweak Synth is $19.01 cheaper than the Numark M6 USB at the latest prices (Nov 9)