I have a Yaris Ativ 1.3 CVT and I have relatives who have 1.5 CVT so here are a few things i have investigated:
1) In city if your route is usually clear and not too much traffic jams are present then 1.5 gives a better fuel average BUT if you have traffic jams and signals in your daily route then definitely the 1.3 will give you better average + if you idle a lot with AC on 1.5s average will drastically drop during that time because 1.3 consumes less fuel than 1.5 while idling (especially with AC on). I live in a busy area so currently i am getting around 10.5-11 (including a lot of idling). During Weekends when there was lockdown i could easily get 12+ in city.
2) On Motorway at the speed of below 100 both provide similar average but after 110 the 1.5 will have a better average. My car gave me 14-15 (full load) at speed of 115-120 (After installing Cruise Control) but before that the max i got was 13.5-13.8 at 120 with 5 people and max luggage
3) I have traveled uptill Babusar Top on my Yaris 1.3, i was satisfied with the performance but occasionally i had to shift to Manual Mode to Keep the car going on steep inclines however in 1.5 you just have to put the Car in Sports Mode and you are good to go.
Conclusion: If you don’t live in a very rushy area with traffic jams and too many traffic signals and you don’t idle a lot then get the 1.5 or if not then get the 1.3.