In terms of voicebanks, Piko's optimum ranges covers 23 notes and Len's 24. Otherwise, if sharps and fat notes are discluded, both have 14 notes each in their best range, which basically means the difference is merely where they fall on the keybaord and which flats and sharpes they cover in their optimum range.
Otherwise, in terms of value in notes and what Len offers, he is the overall more extensive of the two regardless. If one counts what he has assecess to, he has 5 Voicebanks in total versus Piko's 1 and covers a much larger vocal range for this reason across different areas. Even in the base vocal, he is sold alongside Rin, who has a larger vocal range then either Len or Piko. This along with her Act 2 and Append together with Len's 5 voicebanks will provide 10 vocal libraries between them.
Though the Kagamine's have a quality issue, Piko is also not without his own faults. Therefore, the Kagamines are often a much better option then Piko, however, Piko's main advantage was always his more flexiable voicebank compared to some other male Vocaloids.