For what its worth, I've flown both A319/320 and all versions of the 737 except for the NG. I rode in back on CO from SEA to EWR on an 800 this past summer and I was pleasantly suprised. Very comfortable. The Bus is great to fly but after 5000 hours in the left seat I'm ready to try something diffrent. I am tired of fighting with flight standards people who want the automation on all the time. I hand fly all my approaches and I would prefer the HUD for CAT 2 or 3 the way SWA does it. I have been in the J/S for a CAT 1 approach and I think it is great. I have always admired the French Aeropostal guys for hand flying their approaches. Its great to have an autopilot and autoland capabilities for times when you are toast from flying all night but as a standard I would rather not.
Easy Jet should continue with 737s not for the above reasons but because it makes the most sense. As much as I dislike the Herb Turds (SWA) for the type of flying and the type of pax they do I admire them for having the best balance sheet in the business and the foresight to stick to their plan. Boeing did not seem to have a problem giving them discounts when they made their last order. A mixed fleet when you are doing the same type of flying makes little sense. We operate both and it forces all sorts of bad crew schedules operating a mixed fleet into the same cities(a lot of 30 hour layovers for no pay).
As far as safety both are the same, just diffrent issues.