Range is the headline
The Zero SR/S offers up to 187 miles combined range; the S2 Del Mar manages 121 in the city but only 60 on the highway. For highway commuting, Zero is the clear pick.
Weight and feel
The S2 Del Mar is lighter and nimbler, making it more accessible for new riders and more fun in the twisties.
Charging
Both accept Level 2. The SR/S can fast-charge with optional charge tanks; the Del Mar relies on Level 2 home charging.
Verdict
Buy the Zero SR/S for range and touring capability. Buy the S2 Del Mar for a lighter, more affordable city bike.