I advise you not to buy miners because difficult has tripled in the last 6 months ... so you can expect to make at least half of how much you can mine nowadays with for example S5 ,better buy some coins ( I expect the price may go unde 260-250$) and hold long-term 
Isn't there a limit to maximum hash in regards to cost-effectiveness; network pressure will not go over a certain amount as to remain profitable. Considering this, can/will it triple again? I can't tell.
But you make a valid point, I could just buy 6 BTC now and let them sit for 12 months. Maybe they'll hit 500.
Now you put doubt in my head
I'm just saying the facts resulting from this chart http://www.coindesk.com/data/bitcoin-mining-difficulty-time/ and this does not mean that difficult will triple again in the next 6 months and I guess they'll hit much more that 500 this year