I would choose any coin that has margin trades available and a huge orderbook. This way, no matter if the coin is up or down at that point, you can go short or long.
Picks would include bitcoin, eth, litecoin, xrp, etc.
I think margin wouldnt be a good idea. What happens if trend changes during that 7 months? If it goes up then down and up again sth like that you would loose all your profit.