im no expert over this but in my opinion
the sporty one is costly because of alloy wheel i guess and the sporty design
it also have front disk brake , it have self start as well *(citation required)
just have same question in mind search google for specs of three found something highlighted
4 Stroke, 70cc ENGINE
4.2 kW (3000 RIMin ) Power
Payload: approx 150 kg
UD-100
4 Stroke, 100cc ENGINE
7.8 kW (8000 R/Min )Power
DISK BRAKE
SELF START
ALLOY RIM
RPM - METER with FUEL GUAGE
Payload: approx 150 kg
UD-125
4 Stroke, 125cc ENGINE
6.8 kW (8500 R/Min) Power
Payload: approx 250 kg