I find my trunk-style suitcase most useful for transporting items other than clothes that would be too deep to transport in either side of a clam-shell bag. In my case this has included items as large as a classic-size Starlink dish and stand; however, I imagine the same consideration could also apply to certain sports equipment and other non-clothes items.

A trunk style bag is generally more narrow and taller than a clam-shell bag, so that's the tradeoff to consider in terms of space. For public transport, I would imagine that the more narrow footprint would be preferable in an environment such as bus or subway. If your are traveling regularly on trains, you should consider whether the additional height of the bag would make it more difficult to use the normal bag storage spaces (I don't know that it would, I'm just flagging it as an issue to consider). For example, if you were traveling in the type of train car where suitcases are stored in the space between the backs of two seats, you would probably need to lay the bag on its side.