what type of rig would you guys build for 120k ?
I wouldn't put it into hardware. I just buy bitcoins. I could nearly guarantee that in 5 years time it'll be over $100 a pop.
Use it in short trading?
I have no problem dumping 120k into the Bitcoin economy, but if you're looking at a 5 year timeline, is that really the most profitable? Do you have any particular reasons why you think buying BTC is more profitable? Here are 2 scenarios:
Option A: You buy 120k worth of BTC, and sit for 5 years. That's ~3500BTC at today's price, and if you hold on to them for 5 years, they're worth $350K. $120K -> 350K isn't bad.
Option B: You buy 4 BFL ASICs. I don't care if it takes 4 months to deliver, you will have a 100% ROI within the next 8 months. A 100% ROI means you will have mined back what you spent, which would be the same 3500BTC. You then have the next 4 years to mine BTC before you cash out. Lets say the network doubles, and you earn half as much per year. So in the 5 years, you earn 3500, 1750, 875, 437, and finally 218 BTC. At the end of the 5 years, you end with almost 6800BTC. Assuming the same increase in price, you just went from $120k -> $680K.
You can't tell me the cost of power is going to make up the difference between $350K and $680K.
I speak as a miner who has been making decisions (mining/buying/holding/selling) since when the whole network was ~100 Ghash/s... and I have told you this before, but you
how fast the network is going to grow in the next few months and years. "2x per year", yeah right. For reference, the network's mining speed was growing by 100x every ~6 months when GPUs were introduced. Yes. Not 2x, but 100x.
(I also think you overestimate BFL's ability to ship in merely 4 months IMHO). Because of that I don't think that anybody can assert with good confidence that one of the options is definitively more profitable than the other.