Yeah, I understand your reasoning. I was, at first, very skeptical of going with Seagate... but, I got over it after several weeks of research. I've been working w/ HDDs since 'the beginning'. (Really.) But, I'm still new to this this NAS... still learning my way around it's Linux-ness. And, even though I'm new here, I already appreciate the folks on this board. The community interest and support for this device is the main reason I bought one.
Regarding heat, the fan in my DNS-323 rarely runs. Very rarely. Mine currently idles at 95°(F)/35°(C)... that's very low and w/o the fan running. After an hour or more of continuous disk activity, it ultimately reaches 113°(F)/45°(C)... still comparably low; the fan ~finally~ kicks in at this point; and, w/i five minutes, the temp begins to drop. (Still, I'm gonna keep an eye on this.) I'll tune it down to a lower threshold, soon... maybe today. I'm running firmware v1.07 and have no fan control; but, v1.08b claims... "30. Change fan speed design, Shut off fan at desired temp (ability to check always on low, Add always on.)." So, I'd say my experience has been exceptional.
Also, I've read that, out here in the real world, these Seagates actually run a couple of degrees cooler (i.e., during disk activity) than their W.D. counterparts.