Cruisers just don't have the handling, design, ergos to ride aggressively. I also feel the pressure in my lower back after a while. Sport bikes, on the other hand, are generally forcing you into a sitting position that is only good for a race track.
Best all around bikes for longer trips are bikes that were actually designed for it - sport touring and touring.
I did own a cruiser (1999 Magna) and a pure SS (1997 CBR900RR) and neither one really suited my needs. I preferred both the SV650S I had before and the VFR800 I have now. However, if I was forced to pick one at gun point, I'd pick a SS over a cruiser just because it's more capable in terms of performance and handling.
A Magna is actually a Universal Japanese Motorcycle (UJM) and not a cruiser,they began to drop production in the mid 80's When sport tourers came out.Basically a UJM was the first attempt at a compromise between a sport bike and a cruiser.I spent years trying to replace my Midnight Maxim.The only two bikes really left from that segment is the V max and the V rod.The V max replaced the Maxim and does nothing well except go fast.I agree that a sport tourer is the proper choice for the highway but i tourer for weeks on end in the summer and a cruiser is a far better choice than a sport bike(soft ride and plenty of storage) I will ride to BC on a cruiser but cant get out of Ontario on a sport bike and make any time because I have to stop constantly to stretch.