How economical it is depends on how long you need the scooty in Bangalore.

If you are staying for less than 6 months, renting is a better option. You can check out Royal Brothers (~5K/month INR).

If you are staying longer (which I assume is why you are considering both options), buying a second-hand scooty makes more sense. BeepKart is a good option. They have multiple checkpoints, similar to how Spinny works for cars. Affordable ones sell out fast, so you'll need to keep an eye on it. Expect to get one for around 40-45K INR

I know folks who have been using BeepKart scooties and someone who rented for a month before eventually buying a new one.

Riding two-wheelers in Bangalore isn't the safest, but if you really have to, these are the best options I can suggest.