i dont think that BTC difficulty will go up just 50% per month... it will be at least 80% if not more... ASICs are doing grat job in that pump...
anyway im thinking about the same CoinTerra IV - BUT im not sure if the real delivery will be before february or so... every single day delay is cutting down profit and increasing payback time... thats the biggest issue there...
Also consider that they haven't actually ship anything to anyone yet. It is still in production. What if Cointerra closed down? You can only buy either via Bank Transfer or BitPay. There are no chargeback for either of these.
If you really want to mine, I will go with KnC Miner.