Unarguably, Maruti Swift is a better vehicle if compared with Wagon R Stingray in every aspect, be it engine performance, be it mileage, or be it interior space. To draw a detailed comparison between the two, kindly click the below link: -
Maruti Suzuki Wagon R Stingray comes with good headroom, low maintenance cost, easy availability of service center but if you are looking for the car that has safety features such as ABS and dual airbags along with factory fitted LPG kit, then you should go with Hyundai i10. Moreover, base variant of Hyundai i10 is cheaper than that of Wagon R Stingray, but its opposite in the top-end variants.
However, Wagon R ( not the Stingray) comes with the factory fitted LPG and CNG kit.
Maruti Suzuki WagonR Stingray does not comes with diesel engine. However, in the diesel segment, you can look for the Swift and Hyundai Grand i10. To know more about the diesel swift, you can click the link below: