I think the biggest defining factor for the next Bitcoin is "transaction fees". It will even surpass Bitcoin as a real currency wherein people would use it to buy stuff. Bitcoin itself would be hard to be used a real currency because of its high transaction fees. During its early stage, Yes it is being used like that But right now its more of a investment rather than a currency.
If i would choose one its Ripple, great program higher transaction rate, started to be adopted internationally and Low transaction fees.