One thing going for Wow over EJ is the flight timings, especially for weekend breaks - EJ departs at crack of dawn and returns 10ish from KEF, Wow is at more leisurely afternoon/evening hours. Already booked a trip (but did use a credit card!)
As someone with great experience of the Icelandic market with numerous visits through employment and as a tourist, I would be very extremely nervous about booking with IEX right now, and WOW's network looks mighty similar to IEX's....perhaps a bit too conveniently similar.