As Lavdumperer said, from a pax perspective, the A319/320 is a much more comfortable aircraft than the 737 with wider seats/aisle etc and, these days, pax are more type-aware. Flying A319s would give Easy a significant marketing product differentiator with which to beat RyanAir etc over the head. Also while fleet transtion is a pain it doesn't last for ever and, in the overall scheme of things, won't be a major consideration - especially if Airbus a picking up the tab.