I agree that the Mamiya's optics are razor sharp, that and the larger negative will do you well for landscapes.

OTH, if you can handhold the Mamiya at 1/15 or 1/30, you can handhold the Hasselblad at that speed and still get good shots (or not).
Either way, at such slow speeds, most people are skating on thin ice for getting really sharp results. Though there are almost always alternatives for bracing if you don't happen to have a tripod.