Disks are disks, in the end
I have a farm of about 200TB of disk in a variety of arrays, from EMC, DotHill, Pillar and others. The drives range from 7200rpm SATA, in the one of the EMCs, the Pillar and some OEM Adaptec storage, through 10K SCSI in the elderly DotHills and on through 10K FC in the remaing EMCs. We don't do 15K because an 8-way assemblage of 7200rpm is enough performance on the read side and our writes are bursty enough that the battery-backed RAM on the arrays isolates us from write performance.
The reliability appears totally unconnected to the brand and specification of the storage on the sample sizes I have. The Pillar has 100-odd 500GB SATA spindles, and (reaches for large pieces of wood) hasn't dropped a spindle in 18 months, for a truly awesome MTBF. It supports a large Clearcase environment and a whole bunch of Oracle databases, so it takes a pounding. The EMCs, on the other hand, have suitably fairy-dust sprinkled FC drives, and at one point we were on first-name terms with their maintenance people. That, and the hideous botch on Clarrions where the failure of one of the leftmost disks causes the write cache to stop working, made for some entertainment for a few months. We'd had a bad batch: of course you tend to get all the disks in an array you buy from a single batch.
The fun with the EMC SATA is that rebuilding an 8+1 RAID5 group to a hotspare with a centralised RAID controller is Not Quick: the Pillar improves on this (we've tested it) by having a RAID controller on each shelf to it can saturate the hot spare, but nonetheless rebuilding 500GB of parity is not rapid.
But for the Time Capsule, the whole thing seems moot. Unless you have 20 small laptops, the duty cycle for a Time Capsule will be a spin up, some writes and a spin down a few times an hour. In fact, a disk designed for 24x7xpounding may not be the best bet: a drive optimised for frequent spin down, like a laptop drive, might be a better fit. Six 120GB SATA 2.5" laptop drives in a RAID6 assemblage would be the thing, wouldn't it?