OEM doesn't fail to often, mind you they are getting pretty old now, there is a pretty simple testing procedure in the FSM, might be wise to heat the coil up while doing tests to see if temp is effecting the results. Also replacement boots or pull the boot off testing without boots might be wise aswell before replacing the coil(s). Dunno if I would bother with aftermarket here, if I had to I would go with a local parts place.
I just changed my original Sumitomo 1999 wires, and put OEM sumitomo 2020 wires back on, no change runs as good as always if that's any testament. I like that they mfg date them, so NGK or sumitomo OEM, the Sumitomo Wires fit a LOT better than the NGK's I put on my other truck.