You mentioned that people have varying preferences, but don't explain the reason for their preference. I'll try to explaing mine. I'm a civil engineer, and though structural engineering is not my area of particular expertise, drainage(on a larger scale)is. I would recommend using rigid pipe rather than flexible. The reason is that unless the trench to the sump pit is graded extremely precisely (or has a great deal of slope)and the gravel/rock layer is extremely uniform, it is likely that a fexible pipe will have several humps in it along the route to the sump pit which could slow or impede the drainage. While this problem is still possible with a rigid pipe, it is much less likely.