From Verizon's 2015 Data Breach Investigations Report (DBiR):

Before we get too far, let's just get this out of the way now — Android™ wins. Not just wins, but Android wins so hard that most of the suspicious activity logged from iOS devices was just failed Android exploits. So while we’d love to compare and contrast iOS to Android, the data is forcibly limiting the discussion to the latter. Also, the malicious activity recorded on Android is centered on malware, and most of that malware is adnoyance-ware and similar resource-wasting infections.