Since bitcoin has beat paypal in terms of volume it's clear where the potential is... Bitcoin.
Huh?
Got sauce for this wackiness?
Paypal market cap is estimated to be over $50 billion (Paypal market cap was exactly at $46.6 billion in July 2015).
Bitcoin transaction volume last 30 days 53,900,000 BTC = around $23,700,000,000
PayPal transaction volume last 30 days = $26,600,000,000
Bitcoin is very close.