I like the engine of my car way more than the one in belta. Even though my car is on cng and the belta on petrol, acceleration wise both feel the same i'd say but the difference in sound and feel is huge! Vitz is quite and you normally won't hear the engine sound in the cabin while in belta its the opposite.
Underpowered? Well I hope you have driven the city vario and know how a car with CVT drives. What i personally find very annoying is that, in the belta, when you're accelerating and want to reach at a certain speed quickly i.e. 80km/h.. The engine would keep roaring at high rpm until you reach that speed. The sound that it produces in the process makes you want the car to change the gear (But no, its CVT!)
On the other hand I just can't get over the fact that if i buy the new city, i'll no longer be able to retract side mirrors while sitting inside, drive a car worth 13+ lacs with no airbags, driver and passenger map lights etc (A) for the same reason i was looking for a vti oriel 05-06 which offers all these except the airbags. Vitz has ruined me :S
I don't know abt the plugs since its my friends car.
Drive both and decide. You can test drive the belta if you want, i can arrange that for you. But I haven't driven the new city myself.