All easyJet Airbus aircraft are A319's. There are NO A320's in the fleet or on order.
The eJ A319's are re-configured in the galley / toilet areas, and this combined with the seat pitch means 156 seats (but at a more comfortable seat pitch than the 737's).
With more than 150 seats the standard A319 fuselage would not meet evacualtion standards, hence the A320-style double overwing exits. This means the aircraft far exceeds the evacuation standards required as in effect they've fitted 2 additional exits for the sake of 6 seats!
As far as future orders are concerned, eJ have the option to 'upgrade' some of their future orders / options to A320 or 321 in the future. No sign of this happening yet though.
Adding a larger type into the fleet would cause a number of complexities in terms of the basing and rotation of aircraft, upgrade or downgrade of aircraft size due to disruption, etc. so would be difficult to manage in a complex flying programme such as eJ. (Wheras a swap between a 737 and 319 can easily be done with minimal, if any, passenger load impact.)
I suspect that the A320 is unlikely, but possibly the A321 in the future for some of the larger and more slot-constrained operations. (LGW?)