I've never used the purples. I have blacks in personal PC and reds as backups for my NAS devices should any of them fail....
My experience with Seagate HDDs is that they will usually crap out just prior to warranty expiring, if you are lucky....I had them in a Netgear ReadyNas and every one of them died within a couple of months of the warranty expiration date. I think maybe one of them died after expiration date came and went...don't remember.
However, I've always had good performance and reliability from WD HDDs....