Here's how I see it: if you are in Karazhan, you have to wait until you can kill Netherspite to get a significant upgrade from either peices or you can get lucky with a High King Maulgar kill and get T4 (which is an upgrade to Netherspite's).
Now, your results may differ from mine, but it took me about 6-8 weeks of Karazhan with a new guild starting from the first boss to finally get the shoulders from Netherspite (guild's 3rd kill, if I recall), which I replaced 2 weeks later with T4. The entire time, I was waiting with a blue, level 63 piece of gear.
So if your result are going to be similar to mine in time invested before you get your reward, then YES, it would definitely be worth farming for that while you try to get Netherspite down.
The drop rate on the Mantle of Abrahmis from Netherspite is about 8%. I'd try for the T4 if you have access to Gruul's. But its always best to maximize chances so I'd recommend farming all three. If your not in Kara or Gruul's, try to put together a Heroic Seth run.