both cars have pros and cons:
-Santro being taller, its very easy to get in and get out, specially for the elderly people as compared to cultus.
-Sitting on front seats, the comfort level is similar, however the back seats of santro are a bit hard and less comfortable, specially if there are no headrests.(only exec version has those)
-Cultus is definitely more stable at high speeds / highway speeds.
-Santro has better braking,
-I prefer Santro when it comes to engine sound, it produces nice prrrrrr and cabin has better sound proofing, sitting on backseat of cultus, there is annoying generator sound specialy at high speeds.
-Santro has almost a bullet proof engine, if you take care of it, it will go for miles and miles without fail or loosing performance. however g10 is easy to work upon.
-Spares are easily available for both cars.
-there are a few more goodies installed in santro, like factory fitted fog lights, side mirrors adjustment knobs from inside. oh yes, santro has better road visibility ground clearance also better as compared to cultus.
-i found santro tougher than cultus.
for santro i would recomment the hyundai sajjad motors on peco road, as they have all expertise and spares etc.
for cultus, you can goto suzuki township motors.