W520 weighs about as much as a house, 2nd laptop drive doesn't hardly change that.
I will guess that the silentest mechanical drives ought to be single platter 5400 rpm ones, more platters and heads should only mean more energy and noise. But with my limited experience with modern drives I'm neither aware of the silent models nor have I experienced an objectionable 2.5" drive recently. In theory 7200 rpm drives should have less latency in a server application, but bigger drives make up much ground in data density. With my linux systems I don't see gross performance differences between my drives anyway. I booted a live CD to make 2 backups of my system partition; the first backup proceeded at over 100 MB/s, the second was nearly instantaneous because the entire source partition was already cached (!), and the writeback gets cached too, pending forced syncronization.
mSata is brilliant, but IMO need not be considered until you know you need it.