Which gun is more reliable

The 98 is since it is the simplest, but when you figure price, reliability, and performance the ion wins out.

The A-5 and X-7 are basically the same gun. The are slightly less reliable than the 98c IMO, and the cyclone feed is a liability since if it breaks you are out of luck. The ion has more parts that can break (wires and what not), that it less reliable than the overly simple tippmann (basically not as idiot proof). The perfomance and possibilities on an ion far surpass those of the tippmann line of markers.

I suggest a new promaster (possibly the Lasoya model) for a new gun instead of an ion,...

or buying used get a DMLCD or Angel LCD/ LED. Those guns are heavier and might not have eyes (which they dont seem to need), throw down a decent amount of paint, can be found cheaply, require a very small amount of maintenance, are extremeley reliable, and built like a tank.