Personally I wouldn't worry as much about the COPs as I would the spark plugs, you're definitely at the milage for new plugs. My mustang is at 80,000 miles and one if my COPs retaining screw has broken off. I bought a full set of 8 off eBay used (supposedly 30,000 miles) for only $53. My plan is to replace them one by one with my back ups if they go bad. When i first got the car I replaced the plugs and properly gapped them, it definitely made the engine run more smoothly, so I can't say what a bad or worn out COP feels like.
My problem is also money. If you've got the money go ahead, because preventive maintence can save you a lot.
As far as the different brands of COPs go, there's not much performance that can be gained from replacing a good motorcraft one. At least this is all from what I've discovered from looking into replacing mine.