To date, iOS has had exactly NO malware issues.
Meanwhile:
Android malware up 472% since July, unlikely to slow down | Electronista
McAfee: Android malware surges 76%, iPhone untouched | Electronista
However, please note that this is all about how safe the OSes are from malware, and says NOTHING about the quality of the wireless connection you're using.
Bank websites are encrypted, as was previously pointed out, but when using an unsecured or public wireless network there is SOME risk that personal information could be intercepted by someone nearby. The risk is fabulously low, but it exists (PARTICULARLY, I've noticed, at airports!).
Thus, as a rule of thumb, I don't do any banking or credit-cards stuff when using a public or unsecured network. My own home network uses WPA2 and between that and the bank's encrypted connection I'm satisfied that this is sufficient in my situation for banking/financial stuff.