The WD Red drives served me very well, were quiet, and I had no complaints about them at all, so this was really a decision based solely on price (at the time of purchase, the 12TB Gold drives cost $229 each) and capacity. The equivalent Red drives were about $30-50 more a piece than the Golds at the time of purchase, so I went with the Golds.
The conversion took about a week's time of removing a 6TB drive and letting it rebuild the array on the Gold drives (each Gold drive was formatted completely and then had a full SMART scan done on them, none of the 6 drives had any errors or defects).
However, once I converted to having all 6 of the 12TB Golds in my array, I immediately noticed an uptick in the noise level, even during normal operation. The sound of these drives when they seek or perform any kind of scan and/or data scrubbing (all the drives generally aren't going unless data scrubbing or a full SMART scan is running) is much louder than the Reds. I expected an uptick in noise due to them being "Enterprise" (a.k.a. designed for a datacenter) and 7200RPM, but I was surprised at how audible the sound is. The attached video I took to give you an idea of the noise level. The amibiant sound can be ignored (fan hum and sound of a network switch) but the "cachunk"-ing you hear is all the seeking of the drives. I recorded it with my phone on top of the NAS directly in contact with it, and then from about a foot away from the NAS so you can get a sense of the sound. This was taken during normal operation (watching a video off the NAS from a single workstation).
When it's doing data scrubbing you can hear my NAS from the hallway outside the room it sits in. Luckily it sits in an office and not in our bedroom or somewhere guests would normally be. However anyone who may have their NAS or server in their bedroom or near a workstation you use frequently would likely get annoyed by the constant sound of the seeking of these drives. If this sits in your basement or a closet somewhere, you'd probably have less of an issue of the sound. It also may be because I use a NAS rather than a server that might have more sound dampening built into it.
Other than the noise, however, the performance of these drives is great. Not really a noticeable improvement over the Reds, but the price per TB and the extra warranty is awesome.
I still highly recommend these drives, just be wary of the increased noise level, especially if you are moving from a non-Enterprise 5400/5900RPM drive to something like these.