I’m pretty sure that the sonar will work better. The problem with the IR sensors is that their range is very short (normally only 1.5m) while the sonar can be up to 10m. Also the IR sensor beam is very narrow, that’s ok if you have a lot of them on the front of your vehicle but a sonar with it’s wider beam might be better. The maxbotix sonar have a wide variety of beam lengths from very narrow (EZ4) to wide (EZ0).
The Maxbotix sonar are self calibrating for the given environment and much less prone to giving false returns. I speak from experience as I have used both IR and the acoustic sonar.
One thing about sonars is you’ll need to daisy-chain them if you want multiple sonar at the same time or the pings will interfere with each other. Rover is the only frame we have that supports multiple sonar at the moment for object avoidance.