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.