If you are looking to invest over the longer term, I would advise just buying the physical. All platforms that offer bitcoin coin derivatives (be they be CFDs, futures and spreadbets) all seem to charge holding costs at exorbitant rates. At least IG pay you for holding short positions, typically 12.5% (its lower for bitcoin) on nominal value. In fact I'm looking at buying physical ether and completely hedging on IG to eliminate price risk and just earn the 12.5% the short leg.