Stick with whatever plugs it came with. When it's time.
True story: I always used Champions in my 1995 Cherokee. One time I went to my parts store and they stopped carrying them. Said quality has gone downhill yadda yadda yadda and sold me some AC Delcos. No problem for months. Then I drove it from San Jose to Las Vegas -- about an 8 hour drive. Everything was fine until we got off the freeway in Barstow for lunch. Misfiring like you wouldn't believe, six hours from home. Since it didn't seem like it misfired at high speeds, we kept going. Still acted up when we got off the freeway in town. The next day, it was running perfect. Ran perfect for several days. Driving home, it acted up again after a long high speed run. I had the revelation that no matter what AC Delco says, the heat range was wrong. I put in some Champions I bought at Kragens (remember them?) and I never had another problem like that for the next fifteen years I owned it.
If you have a mileage problem, concentrate on the nut behind the steering wheel. That has more effect on economy than anything else you can control.