Hello Dheeraj,
Indeed 4 cylinder Naturally aspirated engines are more smooth as against 3 cylinder one
Now in between
Hyundai Exter Sx Option and
Maruti Fronx Delta Plus - key differences :-
a. Maruti Fronx feels slight more peppier and quick in acceleration response (difference though marginal to be near 1 sec in 0 to 60 and almost 2 sec in 0 to 100)
b. Steering feedback and suspension dynamics are pretty well sorted for city drive conditions like Mumbai in both cars. Still overall handling in terms of wider tyres R16 195 section along with even an extra edge feedback on handling - Fronx has an edge over Exter
b. Though the kind of space you mentioned for 2 adults and 2 kids - both cars meet in need, but still if you seek space as factor - Fronx will end up to be even more spacious with superior legroom perspective. But if rear seat is not deal breaker then Exter with 390 Litre Boot space is more practical as against Fronx 308 Litre Boot. Headroom space is better in Exter too
c. Seats are better cushioned in Fronx as against Exter. Adjustable head restraints on front seats feels better as against integrated one in Exter
d. End Mileage in Fronx would be near 10% better as against Exter
e. Road Presence, Design Language of Fronx feels more catchy been bigger longer, wider car.
f. From Features perspective - Hyundai Exter leads with Sx Option having 6 Airbags (As against 2 in Fronx) along with lifestyle, convenience features - Push Ignition, Rear AC, Electric Sunroof, Digital Instrument Cluster, Rear View Camera, TPMS as extra features giving an edge to Exter
g. Reliability, peace of mind ownership, after sales is very competitive and you will barely encounter an issue. Still Hyundai has a marginal edge been 2nd year service (3rd service is labor free) and even offers warranty choice of upto 7 yrs
Overall
Hyundai Exter Sx - offers more lifestyle, convenience, safety features, bigger practical boot
Maruti Fronx Delta Plus - offers more fun, pep, marginal edge in handling, seating comfort and fuel efficiency with better road presence
Still as you got a small family and need of 2 adults and 2 kids with major city use where rear seat space wont be deal breaker and in city use difference in pickup is marginal - Hyundai Exter Sx can offer superior lifestyle experience and practical car for need.
Have test drive to experience.