It's amazing the range in price we see in spark plugs nowadays. There are so many designs, so many materials, so many brands claiming performance advantages. Honestly, I wouldn't consider any horsepower gains to be realistic. There are different heat ranges, but this should not be a concern with your engine in stock form.
What it comes down to is how long you want the plugs to last. For about $1.00 per plug (Autolite), you'll get a solid 20k miles of service, perhaps a bit more. The OEM brand (AC Delco) are basically the same product, selling for a premium price of $1.65 ea.
Platinum plugs cost more because they last about 5x longer than conventional plugs. This is how GM can offer a suggested 100k mile tune-up interval with its newer cars. Not surprisingly, the price can be $2-5 per plug. There's also an iridium option, even more durable than platinum, selling for between $5-8 each. Iridium plugs can in theory last to 120k miles.
A spark plug's lifespan can depend on the quality of the combustion process and condition of the engine, too. This is more of a concern with older engines, or ones with higher miles. Even if the plugs themselves are resistant to wearing out, if the engine isn't running 100% for some other reason, the plugs may be vulnerable to depositing or fouling, which can shorten the their life.
So the "best" plug may or may not be the one that costs $7.50 and lasts 120k miles. It depends on the condition of your engine, how much you want to spend, and how long you plan on keeping the car. If changed frequently, the $1.00 plug is a great option. Or maybe the best answer is a compromise - a low priced platinum plug in the $1.50-3.00 range that will last a long time but not break the bank.
A list of various plugs that fit your engine, with prices:
http://www.rockauto.com/catalog/raframecatalog.php?carcode=1021992&parttype=7212
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"An intercooled, supercharged, 280 HP Buick Riviera with neck-snapping acceleration and precise handling that proves GM can make something really interesting and competent, if only they would build more than one at a time."
- John R. White, Boston Globe
"I think that in any racing engine, the nearer you are to disintegrating, in general the better its performance will be."
- Keith Duckworth, Cosworth Engineering
'98 SC Riviera • 238k miles • 298 HP/370 LB-FT • 0-60: 5.79s • ET: 13.97 @ 99.28 • 4087 lb • 20.5 avg MPG
3.4" pulley • AL104 plugs • 180º t-stat • FWI w/K&N • 1.9:1 rockers • OR pushrods • LS6 valve springs • SLP headers • ZZP fuel rails
KYB GR2 struts • MaxAir shocks • Addco F/Rsway bars • ES links/bushings • GM strut brace • Enkei 18" EV5s • Dunlop DZ101 tires
F-body calipers • EBC bluestuff/Hawk HP plus • SS lines • slotted discs • ATE superblue fluid • DHP tuned • Aeroforce • Hidden Hitch
