Updated: Jun 5, 2021
Both are wonderful physical modeling synthesizers. (VL1-m is much expensive than VL70-m. They are no longer on production from Yamaha. Used ones are available on ebay). From research on the internet , the main differences are;
1) VL1 has higher clock rate (48k ; VL70-m has 44k) - its sound is deeper , richer (more bottom and high end)
2) VL1 has dual VL voices - it allows more complex, better, thicker sound desgin by layering two voices together
3) VL1 has the impuse expander as VL element, which can make voices more metallic, harsh feeling
I agree VL1 sounds slightly better than VL70-m. But I feel a VL70-m is still sufficient for all sounds that we need for physical modeling synthesis.
I tried other hardware synths : I feel VL synth's DAC(generally speaking, Yamaha synths's DAC) is superior than others such as XpressO, Craftsynth, Roland SE-02, Arturia Microbrute, in my opinion.
So I would recommend a VL70-m as the best EWI hardware synth.