Topeak Bag/Pannier and rack questions

I am trying to get one of Topeak's bags with expandable panniers. I am looking at the MTX Trunkbag EXP.

A few questions.

1. I get the impression that the road bags (RX) are lighter than the mountain (MTX) bags. However, the road racks are stronger and attach more firmly to rear bolt locations than the MTX racks which tend to attach to the seat post. Seems rather backwards!?

2. The expandable pannier bags want some sort of side rack. I'm looking at the explorer tubular rack with disc mount. And the Super Tourist DX tubular rack with disc mount. Will the pannier bags work with either of these racks? Or must I get the "Super tourist" rack?

Thanks.