generally speaking (and I am speaking just for myself) I stick with OEM. I would spend money on a mileage meter. I personally like the scangauge but you can't use that because of the obd-1 or 0 or something. I think there are DIY options though like MPGuino (or something like that)
I don't really think spark plugs are really going to make that big of a difference anyway.
there again, that is my opinion
Be the change you wish to see in the world
my good friend Diamond Larry did some testing of different spark plugs. no appreciable gain from one to the next. true, all vehicles react differently to different mods, but i doubt spark plugs will yield measurable gains in any app.
rule of thumb(for me personally)...
buy the OEM or cheapest plug you can find on 4 cyl cars (and easy to change plugs on 6 and 8 cyl). however, for difficult to reach plugs(like my wife's camaro), i use a long lasting platinum(or similar) plug to avoid changing them as often.