The ME2000 has all the features of the ME1000 plus it uses wavetable synthesis for some instruments.
Instruments like Rock Organ or Square Lead can be easily synthesized using the mathematical synthesis engine of the ME1000. But Piano, Violin, Trumpet and other acoustic instruments require wavetable synthesis in order to achieve a realistic sound. The wavetables requires extra storage in ROM. The wavetable library size can vary from 64KB on up. A typical implementation might use a few hundred KB.
By combining mathematical and wavetable synthesis, we only use wavetables where needed most. This helps us achieve a very high sound quality with a small ROM footprint.
You can hear the difference between the ME1000 and the ME2000 here.