i have both. I flex on return to sump & suction is hard plumbed
i have the sprinkler tubbing for my UV light
I feel more comfortabler w/ hard plumb on suction side/ & like the flex on return. gives u room for change.
I go with all rigid PVC for drain and return. Thus eliminates any chance for leaks. Will as that there is a bit more noise from the return pump doing so but this can be solved by using short piece I flex between the pump and PVC via barbed fitting.
I much prefer a combination of both. Ridgid PVC can lead to vibration and noise problems so using flexible vinyl tubing close to pumps is almost a must. I also like vinyl entering the sump on the overflow as I can shape it in a long sweep or 90 degree elbow so water has a nice smooth laminar flow with few bubbles and noise.
If you go PVC make sure you have plenty of unions for taking things apart for cleaning and maintenance and I also prefer thread by thread bulkheads for the same reason. I have a sticky thread on Bulkhead installation tips that goes in to much more detail.