For me its 50/50 both have their pros and cons and are both roughly the same distance from Lurgan in terms of easy access to get to.
City Airport
Pros
-Its actually in Belfast!.
-Motorway access.
-Its got some decent aircraft (for me anyway).
-The view of Belfast and the apron from the departure lounge.
-Great spotting locations.
Cons
-I find that the price of flights in and out can be expensive.
-Lack of seating in departures.
-Security feels a bit claustrophobic when there are a couple of flights departing roughly at the same time.
International Airport
Pros
-Bigger aircraft.
-Nice selection of destinations.
-Openness of departures.
-There could a great opportunity to lure another north American carrier.
-Plenty of apron space.
-Check-in area is very spacious.
-Security is reasonably quick in my book.
Cons
-BELFAST International?

that's false advertising in my book!
-Charging to drop off someone near the front door (or has this changed?).
-I'm not a smoker but charging £1 to access the air-side smoking area...ridiculous!.
-Other than Kilead church and Ballyrobin road there is no decent photography areas (without having to drive around to find one).
-No big push to entice more new carriers to use the facility.
-The lack of train station near to the airport would detract people not to use the facility (I believe).